typeorm

---

Subscriptions

Subscriptions

Subscriptions are a way to listen for database events and respond to them. This can be useful for keeping your application's data in sync with the database, or for triggering other actions when data changes.

How to Use Subscriptions

To use subscriptions, you first need to create a subscription. This can be done using the SubscriptionManager class.

import { SubscriptionManager } from "typeorm";

const subscriptionManager = new SubscriptionManager();

Once you have a subscription manager, you can create a subscription. The following code creates a subscription that will listen for changes to the users table:

const subscription = subscriptionManager.createSubscription({
  target: "users"
});

The target property specifies the entity that the subscription will listen for changes to.

Once you have created a subscription, you can add event listeners to it. The following code adds an event listener that will print a message to the console whenever a new user is created:

subscription.on("insert", (event) => {
  console.log(`New user created: ${event.entity.name}`);
});

The insert event is triggered whenever a new row is inserted into the users table. The event object contains information about the event, including the entity that was inserted.

You can also add event listeners for other events, such as update, delete, and truncate. See the SubscriptionManager documentation for more information.

Real-World Applications

Subscriptions can be used in a variety of real-world applications, such as:

• Keeping your application's data in sync with the database

• Triggering other actions when data changes, such as sending email notifications or updating other systems

• Monitoring database activity for debugging or performance tuning purposes

Example

The following example shows how to use subscriptions to keep an application's data in sync with the database.

import { SubscriptionManager, EntitySubscriberInterface, InsertEvent } from "typeorm";

class UserSubscriber implements EntitySubscriberInterface<User> {

  listenTo() {
    return User;
  }

  afterInsert(event: InsertEvent<User>) {
    // Update the application's cache with the new user
    cache.set(event.entity.id, event.entity);
  }
}

const subscriptionManager = new SubscriptionManager();
subscriptionManager.addSubscriber(new UserSubscriber());

This example creates a subscription for the User entity. When a new user is created, the afterInsert event listener is triggered. This event listener updates the application's cache with the new user.

---

Query Builder

Query Builder

The Query Builder is a powerful tool that allows you to create complex database queries in a simple and intuitive way. It can be used to perform a variety of tasks, such as:

• Selecting data from a database

• Inserting data into a database

• Updating data in a database

• Deleting data from a database

The Query Builder is based on the concept of a "query object". A query object represents a database query, and it can be used to specify the following:

• The type of query (e.g., select, insert, update, delete)

• The table or tables to be queried

• The columns to be selected, inserted, updated, or deleted

• The conditions to be applied to the query

• The order in which the results should be returned

Once a query object has been created, it can be executed to retrieve the results of the query. The results can then be used to populate an array, an object, or a custom class.

Creating a Query Object

To create a query object, you first need to create a new instance of the QueryBuilder class. You can then use the methods of the QueryBuilder class to specify the various properties of the query object.

For example, the following code creates a new query object that will select all of the columns from the users table:

const queryBuilder = new QueryBuilder('users');

Specifying the Type of Query

The QueryBuilder class provides a number of methods that can be used to specify the type of query. The following table lists the most common methods:

Method	Description
select()	Selects the specified columns from the specified table or tables
insert()	Inserts the specified data into the specified table
update()	Updates the specified data in the specified table
delete()	Deletes the specified data from the specified table

Specifying the Table or Tables to be Queried

The QueryBuilder class provides a number of methods that can be used to specify the table or tables to be queried. The following table lists the most common methods:

Method	Description
from()	Specifies the table or tables to be queried from
join()	Joins the specified table or tables to the current table
leftJoin()	Left joins the specified table or tables to the current table
rightJoin()	Right joins the specified table or tables to the current table

Specifying the Columns to be Selected, Inserted, Updated, or Deleted

The QueryBuilder class provides a number of methods that can be used to specify the columns to be selected, inserted, updated, or deleted. The following table lists the most common methods:

Method	Description
select()	Selects the specified columns from the specified table or tables
insert()	Inserts the specified data into the specified table
update()	Updates the specified data in the specified table
delete()	Deletes the specified data from the specified table

Specifying the Conditions to be Applied to the Query

The QueryBuilder class provides a number of methods that can be used to specify the conditions to be applied to the query. The following table lists the most common methods:

Method	Description
where()	Specifies a condition that must be met in order for the query to return results
andWhere()	Specifies an additional condition that must be met in order for the query to return results
orWhere()	Specifies an alternative condition that can be met in order for the query to return results

Specifying the Order in Which the Results Should be Returned

The QueryBuilder class provides a number of methods that can be used to specify the order in which the results should be returned. The following table lists the most common methods:

Method	Description
orderBy()	Specifies the column or columns by which the results should be ordered
asc()	Specifies that the results should be ordered in ascending order
desc()	Specifies that the results should be ordered in descending order

Executing the Query

Once a query object has been created, it can be executed to retrieve the results of the query. The results can then be used to populate an array, an object, or a custom class.

The following code shows how to execute a query and retrieve the results:

const results = await queryBuilder.execute();

Real World Applications

The Query Builder can be used in a variety of real world applications, such as:

• Retrieving data from a database to display on a web page

• Inserting data into a database when a user submits a form

• Updating data in a database when a user makes a change

• Deleting data from a database when a user requests it

Potential Applications

The Query Builder is a powerful tool that can be used to perform a variety of database operations. It is a valuable tool for anyone who needs to work with databases in Node.js.

Here are some potential applications for the Query Builder:

• Building a CRUD (Create, Read, Update, Delete) application

• Building a data visualization application

• Building a reporting application

• Building a data mining application

• Building a data integration application

---

Configuration

Configuration in TypeORM

TypeORM is an Object-Relational Mapping (ORM) library for Node.js that helps you work with databases in a more convenient way. Configuration allows you to customize TypeORM's behavior to fit your project's needs.

Database Connection Parameters

These settings control how TypeORM connects to your database.

// my-config.ts
module.exports = {
  // ...
  database: {
    type: "postgres", // or 'mysql', 'sqlite', etc.
    host: "localhost",
    port: 5432,
    username: "postgres",
    password: "my-password",
    database: "my-database",
  },
  // ...
};

Entity Configuration

TypeORM uses classes to represent database entities. Entity configuration allows you to define metadata about your entities, such as table names, primary keys, and relationships.

// User.ts
import { Entity, Column, PrimaryGeneratedColumn } from "typeorm";

@Entity("users")
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

Repositories

Repositories are used to interact with your database entities. You can configure repositories to define custom queries, CRUD operations, and more.

// UserRepository.ts
import { EntityRepository, Repository } from "typeorm";
import { User } from "./User";

@EntityRepository(User)
export class UserRepository extends Repository<User> {
  findByName(name: string): Promise<User[]>;
}

Additional Configuration Options

Additional configuration options include:

• Logging: Controls the level of logging output.

• Synchronization: Determines whether TypeORM should automatically update the database schema to match your entities.

• Migrations: Allows you to manage database changes using version-controlled migrations.

• Cache: Enables caching of query results to improve performance.

Real-World Applications

Configuration options are essential for tailoring TypeORM to your specific requirements. For example:

• Database Connection: You can use different settings for development, testing, and production environments.

• Entity Configuration: Define unique table names, set primary keys, and specify relationships to optimize database performance.

• Repository Configuration: Create custom queries and CRUD operations to simplify data retrieval and manipulation.

• Logging Configuration: Set the logging level to "error" in production to only receive error messages.

Improved Code Example

Here's an improved version of the UserRepository example:

import { EntityRepository, Repository } from "typeorm";
import { User } from "./User";

@EntityRepository(User)
export class UserRepository extends Repository<User> {
  async findByName(name: string): Promise<User[]> {
    return this.createQueryBuilder("user")
      .where("user.name = :name", { name })
      .getMany();
  }
}

This improved example uses a query builder to create a more efficient query.

---

Transactions

Transactions

In a database, a transaction is a sequence of operations that are treated as a single unit. This means that either all of the operations in the transaction are completed successfully, or none of them are. Transactions are used to ensure that data is not left in an inconsistent state.

When making changes to data, it is important to do so in a way that protects the integrity of the data. For example, if you are transferring money between two accounts, you need to ensure that the total amount of money in the system remains the same. If the transfer fails, the money should not be removed from one account but not added to the other.

Transactions are used to ensure that data integrity is maintained. When you start a transaction, the database locks the data that is being modified. This prevents other users from modifying the data while the transaction is in progress. Once the transaction is complete, the database unlocks the data.

How to Use Transactions

To use transactions in Node.js with TypeORM, you use the TransactionManager. The TransactionManager provides methods for starting, committing, and rolling back transactions.

// Create a transaction manager
const transactionManager = connection.createTransactionManager();

// Start a transaction
await transactionManager.begin();

// Make changes to data
const user = await userRepository.findOne(1);
user.name = 'New Name';

// Save changes
await userRepository.save(user);

// Commit the transaction
await transactionManager.commit();

If you need to perform multiple operations in a transaction, you can create a transaction scope. A transaction scope ensures that all of the operations in the scope are completed successfully, or none of them are.

// Create a transaction scope
await transactionManager.begin();

// Make changes to data
const user = await userRepository.findOne(1);
user.name = 'New Name';
await userRepository.save(user);

// If any of the operations in the scope fails, the transaction will be rolled back
const product = await productRepository.findOne(1);
product.name = 'Invalid Name';
await productRepository.save(product);

// Commit the transaction
await transactionManager.commit();

Potential Applications

Transactions are used in a variety of real-world applications, including:

• Banking

• E-commerce

• Inventory management

• Healthcare

In any application where it is important to maintain data integrity, transactions can be used to ensure that data is not left in an inconsistent state.

---

Concurrency Control

Concurrency Control in Node.js TypeORM

Concurrency control is a mechanism that ensures that multiple operations on the same data do not interfere with each other. In TypeORM, this is achieved through the use of locks.

Types of Locks

TypeORM supports two types of locks:

• Pessimistic locks: Prevent other transactions from accessing the locked data.

• Optimistic locks: Allow other transactions to access the locked data, but will throw an error if the data has been modified.

Using Concurrency Control

To use concurrency control in TypeORM, you can use the @Lock decorator. This decorator takes an optional mode parameter, which can be set to either PessimisticLock or OptimisticLock.

For example, the following code uses a pessimistic lock to prevent other transactions from accessing the Post entity while it is being updated:

@Entity()
export class Post {

    @PrimaryGeneratedColumn()
    id: number;

    @Column()
    title: string;

    @Column()
    content: string;

    @VersionColumn()
    version: number;
}

@Controller()
export class PostController {

    @Lock(PessimisticLock)
    async updatePost(id: number, title: string, content: string) {
        const post = await this.postService.findOne(id);
        post.title = title;
        post.content = content;
        await this.postService.save(post);
    }
}

In this example, the @Lock decorator is applied to the updatePost method. This means that when this method is called, a pessimistic lock will be acquired on the Post entity with the specified ID. This will prevent other transactions from accessing the entity until the lock is released.

Potential Applications

Concurrency control is essential for applications that require multiple users to be able to access and modify data at the same time. Some potential applications include:

• E-commerce websites: To ensure that multiple users can add items to their shopping carts at the same time without the risk of items being duplicated or lost.

• Banking applications: To ensure that multiple users can make withdrawals and deposits at the same time without the risk of overdrafting their accounts.

• Social media applications: To ensure that multiple users can post and comment on the same content at the same time without the risk of duplicate posts or comments.

---

Introduction to TypeORM

Introduction to TypeORM

TypeORM is an object-relational mapper (ORM) for Node.js that helps you work with databases. It makes it easier to manage your data by converting objects into database tables and back.

Basic Concepts

• Entity: A class that represents a database table.

• Column: A property of an entity that represents a column in the database table.

• Repository: A class that provides methods for CRUD (create, read, update, delete) operations on entities.

Installation and Setup

To install TypeORM, run the following command:

npm install typeorm

Once installed, you need to create a configuration file called ormconfig.json in your project directory. This file tells TypeORM how to connect to your database.

{
  "type": "mysql",
  "host": "localhost",
  "port": 3306,
  "username": "root",
  "password": "password",
  "database": "mydb",
  "synchronize": true
}

Creating Entities

An entity is a class that represents a database table. You define entities by using decorators.

import { Entity, Column, PrimaryGeneratedColumn } from "typeorm";

@Entity()
class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

Creating Repositories

A repository provides methods for CRUD operations on entities. You can create repositories by using the getRepository() method of the EntityManager.

import { EntityManager } from "typeorm";

const userRepository = entityManager.getRepository(User);

Using TypeORM

Once you have created entities and repositories, you can use TypeORM to manage your data.

Create: To create a new entity, use the save() method of the repository.

const user = new User();
user.name = "John Doe";
await userRepository.save(user);

Read: To read an entity, use the findOne() method of the repository.

const user = await userRepository.findOne(1);

Update: To update an entity, use the save() method of the repository.

user.name = "Jane Doe";
await userRepository.save(user);

Delete: To delete an entity, use the delete() method of the repository.

await userRepository.delete(1);

Potential Applications

TypeORM can be used in a variety of applications, including:

• CRUD operations on entities

• Data validation and transformation

• Querying and filtering data

• Managing relationships between entities

---

Running Migrations

What are Migrations?

Migrations are a way to keep track of changes to your database over time. Think of them like a logbook that records every update or change you make. This helps you maintain the integrity of your database and ensures that everyone working on the project is on the same page.

How do Migrations Work in TypeORM?

TypeORM provides a convenient way to manage migrations using Node.js. It allows you to create, run, and revert migrations so that you can easily update your database schema.

Creating a Migration

To create a migration, use the typeorm migration:create command followed by a name for the migration.

typeorm migration:create AddUserTable

This command will create a migration file with the given name in your project's /migrations directory.

Running a Migration

Once you have created a migration, you can run it using the typeorm migration:run command. This command will apply the changes defined in the migration to your database.

typeorm migration:run

Reverting a Migration

If you need to undo a migration, you can use the typeorm migration:revert command. This command will revert the changes made by the specified migration.

typeorm migration:revert

Real-World Applications

Migrations are useful in a variety of real-world applications:

• Adding new columns to existing tables

• Renaming or removing columns

• Changing column types

• Creating or dropping tables

Complete Code Implementation

Here is a complete example of using migrations in a Node.js project:

// migration.ts
import { MigrationInterface, QueryRunner } from "typeorm";

export class AddUserTable1646717552000 implements MigrationInterface {
    public async up(queryRunner: QueryRunner): Promise<void> {
        await queryRunner.query(`
            CREATE TABLE "users" (
                "id" SERIAL PRIMARY KEY,
                "name" VARCHAR(255) NOT NULL
            )
        `);
    }

    public async down(queryRunner: QueryRunner): Promise<void> {
        await queryRunner.query(`DROP TABLE "users"`);
    }
}

Usage:

typeorm migration:create AddUserTable
typeorm migration:run

This will create a new table called "users" with two columns: "id" and "name".

---

Integration with Express

Integration with Express

Express is a popular web framework for building Node.js applications. TypeORM can be easily integrated with Express to enable object-relational mapping (ORM) capabilities.

Configuration with Express

To configure TypeORM with Express, follow these steps:

1. Install TypeORM using npm install typeorm.

2. Create a config file (e.g., ormconfig.json) to define database credentials and connection settings.

3. In your Express application, import TypeORM and establish a connection to the database in the initialization code.

import * as express from 'express';
import { createConnection } from 'typeorm';

// Initialize Express app
const app = express();

app.use(express.json());

// Establish database connection
createConnection().then(async () => {
  // Now your application can use TypeORM to interact with the database
});

// Start the Express application
app.listen(3000, () => {
  console.log('Server is listening on port 3000');
});

ORM Operations

Once TypeORM is configured, you can use its ORM capabilities within your Express application.

• Entity Definitions: Create entities that represent your database tables and define their properties and relationships.

• Repositories: Create repositories for each entity to access and manipulate data in the database.

• Retrieving Data: Use repositories to find, filter, and retrieve data from the database.

• Saving Data: Use repositories to insert, update, and delete data in the database.

Real-World Code Example

Here's a simple code example illustrating how to integrate TypeORM with Express:

// Import Express and TypeORM
import * as express from 'express';
import { createConnection, getRepository } from 'typeorm';

// Entity definition
class User {
  id: number;
  name: string;
  age: number;
}

// Main application
const app = express();
app.use(express.json());

// Database connection establishment
createConnection().then(async () => {
  const userRepository = getRepository(User);

  // Endpoint to retrieve all users
  app.get('/users', async (req, res) => {
    const users = await userRepository.find();
    res.json(users);
  });

  // Endpoint to create a new user
  app.post('/users', async (req, res) => {
    const newUser = userRepository.create(req.body);
    await userRepository.save(newUser);
    res.json(newUser);
  });
});

// Start the Express application
app.listen(3000, () => {
  console.log('Server is listening on port 3000');
});

Potential Applications

TypeORM and Express integration enables various applications in real-world scenarios:

• Database-Driven Applications: Build applications where data storage and retrieval are crucial.

• CRUD Operations (Create, Read, Update, Delete): Implement user interfaces that allow users to interact with data.

• Entity Relationships: Create complex database models with relationships between entities.

• Data Validation: Ensure data integrity by defining constraints and validation rules on entities.

---

Creating Migrations

Creating Migrations

What are migrations?

Migrations are like updates for your database schema. They allow you to make changes to your database over time, without losing any data.

Why use migrations?

• To keep track of changes to your database

• To ensure that everyone on your team is using the same database schema

• To make it easy to roll back changes if necessary

How to create a migration

To create a migration, use the typeorm migration:create command. This will create a new migration file in the migrations directory of your project.

The migration file will contain a up function that contains the changes you want to make to the database, and a down function that contains the changes you need to make to roll back the migration.

For example, the following migration file adds a new column to the users table:

import { MigrationInterface, QueryRunner } from "typeorm";

export class AddColumnToUsers1624972024859 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.addColumn("users", {
      name: "age",
      type: "int",
    });
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.dropColumn("users", "age");
  }
}

How to run migrations

To run migrations, use the typeorm migration:run command. This will run all of the migrations that have not yet been run.

Real-world applications

Migrations are used in a variety of real-world applications, such as:

• Adding new features to an existing application

• Changing the data structure of an existing database

• Fixing bugs in an existing database

• Upgrading to a new version of a database software

---

Data Types

Data Types in TypeORM

TypeORM is an ORM (Object-Relational Mapping) library for Node.js that allows developers to work with databases in a more object-oriented way. It supports various data types to represent data in database tables.

Primitive Data Types

• String: A sequence of characters. Example: "Hello World"

• Number: A numeric value. Example: 1234

• Boolean: A logical value, either true or false. Example: true

• Date: A point in time. Example: new Date()

• Buffer: A container for binary data. Example: Buffer.from('Hello World')

Complex Data Types

• Array: A collection of elements of the same type. Example: [1, 2, 3]

• Object: A collection of key-value pairs. Example: { name: "John", age: 30 }

• JSON: A string representing a complex data structure. Example: JSON.stringify({ name: "John", age: 30 })

• Enum: A predefined list of values. Example: ['red', 'green', 'blue']

Custom Data Types

You can define your own custom data types by creating a class and decorating it with the @TypeORM.Type() decorator.

Example:

// Define a custom data type
@TypeORM.Type('json')
export class MyCustomType {
  value: string;
}

Entity-Specific Data Types

TypeORM supports entity-specific data types that are only applied to a specific entity.

Example:

// Define an entity with a custom timestamp
@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column({ type: 'timestamp with time zone', default: 'now()' })
  createdAt: Date;
}

Real-World Examples

• String: Used for storing text data, such as names, addresses, and descriptions.

• Number: Used for storing numeric data, such as IDs, quantities, and prices.

• Boolean: Used for storing logical values, such as whether a user is active or not.

• Date: Used for storing timestamps, such as when a user was created or a transaction was processed.

• Array: Used for storing collections of similar data, such as a list of tags or a set of permissions.

• Object: Used for storing complex data structures, such as user profiles or configuration settings.

Potential Applications

• User Management System: Use String for storing user names and passwords, Number for storing user IDs, Boolean for indicating user status, and Date for tracking user registration dates.

• E-commerce Website: Use Number for storing product prices and quantities, String for storing product descriptions, and Array for storing product categories.

• Social Media Platform: Use String for storing user posts and comments, Date for tracking post creation times, and Enum for defining post types (e.g., text, image, video).

---

Integration with Next.js

Integrating with Next.js

1. Server-Side Rendering (SSR)

• Allows you to render pages on the server before sending them to the client.

• Improves SEO and initial page load performance.

Code:

// pages/index.tsx
import { getServerSideProps } from 'next'
import { getConnection } from 'typeorm'
import { User } from '../entities/User'

export const getServerSideProps = async () => {
  const connection = await getConnection()
  const users = await connection.getRepository(User).find()
  return {
    props: {
      users,
    },
  }
}

2. Client-Side Querying

• Allows you to fetch data directly from the database on the client-side.

• Useful for performance optimizations and dynamic updates.

Code:

// pages/index.tsx
import { useEffect, useState } from 'react'
import { getConnection } from 'typeorm'
import { User } from '../entities/User'

export default function Home() {
  const [users, setUsers] = useState<User[]>([])

  useEffect(() => {
    const connection = getConnection()
    const fetchUsers = async () => {
      const users = await connection.getRepository(User).find()
      setUsers(users)
    }
    fetchUsers()
  }, [])

  return (
    <div>
      {/* Render users */}
    </div>
  )
}

3. Database Configuration

• Set up the database connection for both development and production environments.

• Ensure your environment variables are secure.

Code:

Development (next.config.js):

const { createConnection } = require('typeorm')

module.exports = {
  async rewrites() {
    return [
      {
        source: '/api/:path*',
        destination: '/api-json/:path*',
      },
    ]
  },
  // ...
  webpack(config) {
    config.module.rules.push({
      test: /\.tsx?$/,
      loader: 'eslint-loader',
      enforce: 'pre',
      exclude: /node_modules/,
    })

    return config
  },
  async serverRuntimeConfig() {
    const connection = await createConnection({
      type: 'postgres',
      host: 'localhost',
      port: 5432,
      username: 'postgres',
      password: 'my_password',
      database: 'my_database',
      synchronize: true,
      logging: 'all',
    })

    return {
      connection,
    }
  },
  // ...
}

Production (vercel.json):

{
  "env": {
    "DATABASE_URL": "postgres://username:password@host:port/database"
  }
}

4. Real-World Applications

• E-commerce: Manage user profiles, product catalogs, and order history.

• Social media: Store user posts, comments, and connections.

• Content management systems: Manage blog posts, pages, and media files.

---

Validation

Validation in Node.js TypeORM

Validation ensures that the data you store in your database is correct and consistent. TypeORM provides several ways to validate data, including:

1. Model Validation:

Decorate your model properties with validation decorators, such as @Column, @Min, and @Max. This validates data at the model level before it is saved to the database.

Example:

import { Column, Min, Max } from "typeorm";

export class User {
  @Column()
  @Min(18)
  @Max(120)
  age: number;
}

2. Query Validation:

Validate data before executing queries using createQueryBuilder() and validate() method. This ensures that the data is valid even if it is not bound to a model.

Example:

const queryBuilder = connection.createQueryBuilder()
  .insert()
  .into(User)
  .values({ age: 15 });

queryBuilder.validate() // throws an error due to the invalid age

3. Custom Validation:

Define custom validation rules using the registerValidationFunction() method. This allows you to create complex validation logic for specific scenarios.

Example:

import { ValidationOptions } from "typeorm";

ValidationOptions.registerValidationFunction("myCustomRule", (value) => {
  // Implement custom validation logic
});

export class User {
  @Column()
  @myCustomRule()
  customField: string;
}

Real-World Applications:

• Enforcing data integrity: Validation ensures that data meets expected criteria, reducing errors and improving data quality.

• Protecting against malicious input: Validating user input can prevent malicious or invalid data from being stored in the database.

• Ensuring consistency: Validating data across multiple models and queries ensures that data is consistent and adheres to application rules.

---

Inheritance Table Per Hierarchy

Sure, here is a simplified explanation of the Inheritance Table Per Hierarchy topic from Node.js TypeORM:

What is Inheritance Table Per Hierarchy (TPH)?

Inheritance Table Per Hierarchy (TPH) is a strategy for mapping inheritance relationships in an object-relational database. In TPH, each subclass has its own table, and the parent class has a column that references the subclass table.

Example:

Let's say we have a Person class and two subclasses, Employee and Customer. We can map these classes to a database using TPH as follows:

@Entity()
class Person {

  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  // ...
}

// Employee is a subclass of Person
@Entity()
class Employee extends Person {

  @Column()
  salary: number;

  // ...
}

// Customer is a subclass of Person
@Entity()
class Customer {

  @Column()
  discount: number;

  // ...
}

In this example, the Person class has a type column that references the subclass table. This allows us to store different types of people in the same database table.

Benefits of TPH:

• Easy to implement

• Supports multiple levels of inheritance

• Efficient for queries that involve a single subclass

Drawbacks of TPH:

• Can lead to data duplication

• Difficult to add new subclasses

Real-World Applications of TPH:

• Storing different types of users in a user management system

• Storing different types of products in an e-commerce system

• Storing different types of documents in a document management system

Code Snippets:

Here is a code snippet that shows how to use TPH in Node.js TypeORM:

// Import the TypeORM package
import { Entity, PrimaryGeneratedColumn, Column } from "typeorm";

// Define the Person class
@Entity()
class Person {

  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  // ...
}

// Define the Employee class
@Entity()
class Employee extends Person {

  @Column()
  salary: number;

  // ...
}

// Define the Customer class
@Entity()
class Customer {

  @Column()
  discount: number;

  // ...
}

Conclusion:

Table Per Hierarchy is a simple and effective strategy for mapping inheritance relationships in an object-relational database. It is easy to implement and supports multiple levels of inheritance. However, it can lead to data duplication and can be difficult to add new subclasses.

---

Integration with React

Introduction

React and TypeORM are two popular tools for building web applications. React is a JavaScript library for building user interfaces, while TypeORM is an object-relational mapper (ORM) that helps you interact with a relational database. By integrating React and TypeORM, you can create dynamic and data-driven web applications.

Installation

To integrate React and TypeORM, you will need to install both libraries. You can do this using npm:

npm install react typeorm

Creating a Database Connection

Once you have installed React and TypeORM, you will need to create a database connection. You can do this by creating a new TypeORM connection instance:

import {createConnection} from "typeorm";

const connection = await createConnection({
  type: "mysql",
  host: "localhost",
  port: 3306,
  username: "root",
  password: "password",
  database: "mydb"
});

Creating a React Component

Next, you will need to create a React component that will use TypeORM to interact with the database. You can do this by creating a new React class or function component:

import React, {useState, useEffect} from "react";
import {createConnection} from "typeorm";

const MyComponent = () => {
  const [users, setUsers] = useState([]);

  useEffect(() => {
    const connection = createConnection({
      type: "mysql",
      host: "localhost",
      port: 3306,
      username: "root",
      password: "password",
      database: "mydb"
    });

    connection.query("SELECT * FROM users").then((results) => {
      setUsers(results);
    });
  }, []);

  return (
    <div>
      {users.map((user) => (
        <p key={user.id}>{user.name}</p>
      ))}
    </div>
  );
};

export default MyComponent;

Using the Component

Finally, you can use the React component in your application. You can do this by rendering the component in the main part of your application:

import MyComponent from "./MyComponent";

const App = () => {
  return (
    <div>
      <MyComponent />
    </div>
  );
};

export default App;

Real-World Applications

Integrating React and TypeORM can be useful for a variety of real-world applications, such as:

• Building a CRUD (create, read, update, delete) application

• Displaying data from a database on a web page

• Creating a user management system

• Developing a data-driven application

Code Snippets

Here are some code snippets that illustrate how to use React and TypeORM together:

// Create a new user
await connection.query("INSERT INTO users (name) VALUES (?)", ["John Doe"]);

// Update a user
await connection.query("UPDATE users SET name = ? WHERE id = ?", ["Jane Doe", 1]);

// Delete a user
await connection.query("DELETE FROM users WHERE id = ?", [1]);

Conclusion

Integrating React and TypeORM is a powerful way to build dynamic and data-driven web applications. By using TypeORM to interact with a database, you can easily retrieve, create, update, and delete data. React allows you to build user interfaces that are interactive and responsive. By combining these two libraries, you can create web applications that are both powerful and user-friendly.

---

Contributing to TypeORM

Contributing to TypeORM

1. Reporting Issues

If you find a bug or have a question, create an issue on GitHub. Provide as much detail as possible, including the steps to reproduce the problem.

Example: Issue Title: "Error when saving entity with null property"

2. Submitting Pull Requests (PRs)

If you want to contribute code or features:

• Fork the TypeORM repository on GitHub.

• Create a new branch for your changes.

• Write code that follows the TypeORM coding style.

• Run tests to ensure your changes don't break anything.

• Submit a PR against the main branch.

Example: PR Title: "Fix error when saving entity with null property"

3. Coding Style

• Use TypeScript for all new code.

• Follow the TypeORM coding guidelines (available in the repository).

Example: Code Snippet:

// Good coding style: explicit types, clear naming
const user: User = {
  id: 1,
  name: 'John Doe',
};

// Bad coding style: implicit types, unclear naming
const person = {
  1,
  'John Doe',
};

4. Unit Tests

• Write unit tests for any new code.

• Use Jest as the testing framework.

Example: Test Snippet:

import { User } from 'typeorm';

describe('User Entity', () => {
  it('should save a user', async () => {
    const user = new User();
    await user.save();
    expect(user.id).toBeGreaterThan(0);
  });
});

5. Potential Applications

TypeORM can be used in various real-world applications, such as:

• Database-driven web applications: Use TypeORM to manage database connections, create entities (models), and perform CRUD operations.

• ORM frameworks: TypeORM can serve as the ORM layer for existing frameworks or build your own.

• Data migration tools: Create scripts to migrate data between different database versions or systems.

Example: Code snippet for a web application:

// Import TypeORM and the Entity class
import { Entity, Column, PrimaryGeneratedColumn } from 'typeorm';
import { Request, Response } from 'express';

// Define an Entity called "User"
@Entity()
class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

// Create an API endpoint to retrieve users
const getUsers = async (_req: Request, res: Response) => {
  // Get all users from the database
  const users = await User.find();

  // Send the users as JSON response
  res.json(users);
};

---

Custom Repository

Custom Repository in TypeORM

What is a Custom Repository?

A Custom Repository is a way to extend the functionality of TypeORM's built-in repositories. It allows you to create custom methods and queries that are specific to your application.

Benefits of Using Custom Repositories:

• Code reuse and maintainability: You can organize custom queries and methods in a single location.

• Extensibility: You can add new methods and queries without modifying the original repository.

How to Create a Custom Repository:

1. Create a class that extends the EntityRepository class and add your custom methods.

2. Register the custom repository in the entityMetadata.ts file.

Example:

// CustomRepository.ts
import { EntityRepository } from "typeorm";
import { User } from "./User";

// Create a custom repository for the User entity
@EntityRepository(User)
export class UserRepository extends EntityRepository<User> {
  // Define a custom method to retrieve all active users
  async findActiveUsers() {
    return await this.createQueryBuilder("user")
      .where("user.isActive = :isActive", { isActive: true })
      .getMany();
  }
}

// entityMetadata.ts
import { EntityManager } from "typeorm";
import { UserRepository } from "./CustomRepository";

// Register the custom repository
export const CustomRepository = {
  UserRepository,
};

Real-World Applications:

• Creating custom queries to fetch data based on specific criteria.

• Implementing complex business logic related to database operations.

• Providing a consistent and organized way to access data from multiple entities.

Potential Applications of Custom Repositories:

• Building a user management system with custom queries to retrieve users based on role or permissions.

• Creating an e-commerce application with custom methods to handle order processing and inventory management.

• Developing a data analysis dashboard with custom queries to generate reports and visualizations.

---

TypeORM Release Notes

Simplified TypeORM Release Notes

Improved Entity Relationships

• New Join Tables: Define relationships using separate tables for many-to-many associations.

• Improved One-to-Many Relationships: Better support for cascade operations and creating new child entities.

Enhanced Query Builder

• Subquery Support: Use subqueries to retrieve nested or filtered data in your queries.

• Improved Raw Queries: More flexibility and control over raw SQL queries, including parameter binding.

• Query Validation: Validate your queries before executing them, reducing potential errors.

Advanced Features

• Multi-Tenancy: Set up separate databases or schemas for different tenants.

• Asynchronous Operations: Perform database operations asynchronously without blocking your application.

• Tree Structures: Model hierarchical data using tree-like structures, such as company departments.

Real World Applications

Example 1: Join Tables for Many-to-Many Relationships

Consider a social media application. You can use join tables to define the relationship between users and their posts:

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @ManyToMany(() => Post, post => post.users)
  posts: Post[];
}

@Entity()
export class Post {
  @PrimaryGeneratedColumn()
  id: number;

  @ManyToMany(() => User, user => user.posts)
  users: User[];
}

@Entity()
export class UserPost {
  @PrimaryColumn()
  userId: number;

  @PrimaryColumn()
  postId: number;
}

Example 2: Subquery Support in Queries

In an e-commerce application, you can use subqueries to retrieve product categories that have at least 1 product with a price above $100:

const query = await manager.createQueryBuilder()
  .select("category.name")
  .from(Category, "category")
  .where("category.id IN (SELECT categoryId FROM Product WHERE price > 100)")
  .getMany();

Example 3: Asynchronous Operations

In a high-traffic web application, you can prevent blocking by performing database operations asynchronously:

const result = await manager.execute("MyQuery", {param: value});
console.log(result); // Do something with the result later

Benefits of these Features

• Flexibility and Scalability: Join tables allow for more complex relationships and multi-tenancy supports large-scale applications.

• Improved Performance: Subqueries and asynchronous operations optimize query execution and reduce response times.

• Enhanced Data Management: Tree structures simplify hierarchical data modeling and tree traversal queries.

---

Deleting Records

Deleting Records in TypeORM

What is deleting Records?

Deleting records means removing data from your database. You might want to do this if the data is incorrect, outdated, or no longer needed.

How to delete records in TypeORM:

There are two ways to delete records in TypeORM:

1. Using the remove() method:

import { Repository } from "typeorm";

class MyEntity {
  id: number;
  name: string;
}

const repository = getRepository(MyEntity);
const entity = new MyEntity();
repository.remove(entity);

The remove() method takes an entity as an argument and removes it from the database.

2. Using the delete() method:

import { DeleteResult } from "typeorm";

class MyEntity {
  id: number;
  name: string;
}

const repository = getRepository(MyEntity);
repository.delete({ id: 1 });

The delete() method takes a query object as an argument and removes all records that match the query.

Real-world example:

You might need to delete records if:

• A customer unsubscribes from your email list

• A product is discontinued

• You need to clean up old data

Potential applications:

• Data management

• User management

• Inventory management

---

Integration with TypeScript

Integration with TypeScript

TypeScript is a superset of JavaScript that adds static type checking. This can help to improve the quality of your code and make it easier to maintain.

To use TypeORM with TypeScript, you need to install the @types/typeorm package. This package contains the TypeScript definitions for TypeORM, which will allow your TypeScript compiler to check the types of your code.

Once you have installed the @types/typeorm package, you can import TypeORM into your TypeScript code. The following example shows how to import the Entity and Column decorators from TypeORM:

import { Entity, Column } from "typeorm";

You can then use the Entity and Column decorators to define your entities and their properties. The following example shows how to define a simple User entity:

@Entity()
export class User {

    @PrimaryGeneratedColumn()
    id: number;

    @Column()
    name: string;

    @Column()
    email: string;

}

Once you have defined your entities, you can use TypeORM to create a database connection and perform CRUD operations on your entities. The following example shows how to create a database connection using TypeORM:

import { createConnection } from "typeorm";

createConnection({
    type: "mysql",
    host: "localhost",
    port: 3306,
    username: "root",
    password: "",
    database: "test",
    entities: [
        User
    ]
}).then(async connection => {

    let user = new User();
    user.name = "John Doe";
    user.email = "[e-mail]";

    await connection.manager.save(user);

    console.log("User saved successfully.");

}).catch(error => {
    console.error("Error:", error);
});

Potential applications in real world

TypeORM can be used in a variety of real-world applications, including:

• Web development: TypeORM can be used to develop web applications that interact with a database.

• Mobile development: TypeORM can be used to develop mobile applications that interact with a database.

• Desktop development: TypeORM can be used to develop desktop applications that interact with a database.

• Data analysis: TypeORM can be used to analyze data from a database.

• Machine learning: TypeORM can be used to train machine learning models on data from a database.

---

Many-to-One Relationships

Many-to-One Relationships

Imagine you have a school with students and teachers. Each student can have one teacher, but each teacher can have multiple students. This is a many-to-one relationship.

Types of Many-to-One Relationships

• ManyToOne: Specifies that the property represents a many-to-one relationship.

• JoinColumn: Specifies the column in the target entity that establishes the relationship.

Code Snippets

// Student entity
@Entity()
export class Student {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @ManyToOne(() => Teacher, teacher => teacher.students)
  @JoinColumn()
  teacher: Teacher;
}

// Teacher entity
@Entity()
export class Teacher {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @OneToMany(() => Student, student => student.teacher)
  students: Student[];
}

Real-World Examples

• Students and Teachers: As described above.

• Orders and Customers: Each order is placed by one customer, but each customer can place multiple orders.

• Posts and Blogs: Each post belongs to one blog, but each blog can have multiple posts.

Potential Applications

• Hierarchical structures: Represent relationships between objects in a hierarchical manner, such as employees and managers.

• Lookup tables: Store additional information about related entities, such as customer addresses or product categories.

• Data integrity: Ensure that related entities are consistent and updated accurately.

Simplified Explanation

In a many-to-one relationship, one object (the "one" side) has a unique relationship with another object (the "many" side). Each object on the "many" side can be associated with exactly one object on the "one" side. This is helpful for representing real-world scenarios where objects have a specific relationship with each other.

---

Raw SQL Queries

Raw SQL Queries in TypeORM

What is a raw SQL query?

A raw SQL query is a query that you write directly in SQL, instead of using TypeORM's query builder. This can be useful when you need to perform a complex query that is not supported by the query builder, or when you want to optimize performance.

How to write a raw SQL query in TypeORM:

To write a raw SQL query in TypeORM, you use the createQueryBuilder() method of the EntityManager class. This method takes a string as its first argument, which is the SQL query that you want to execute. You can also pass parameters to the query using the setParameter() method.

The following code shows how to write a raw SQL query to select all of the users in a database:

const entityManager = getEntityManager();
const users = await entityManager.createQueryBuilder()
  .select()
  .from('users')
  .where('name = :name', { name: 'John Doe' })
  .execute();

Potential applications of raw SQL queries

Raw SQL queries can be used in a variety of real-world applications, including:

• Performing complex queries that are not supported by the query builder

• Optimizing performance by using more efficient SQL queries

• Interacting with legacy databases that do not support modern query builders

Best practices for writing raw SQL queries

• Always use prepared statements to prevent SQL injection attacks.

• Use named parameters to make your queries more readable and maintainable.

• Test your queries thoroughly to ensure that they return the expected results.

---

Migration

Migration

What is Migration?

Migration is like updating your software without losing your data. It's a way to make changes to your database structure over time.

Why is Migration Important?

As your application grows, you may need to make changes to your database. Without migration, you would have to manually make these changes, which could be time-consuming and error-prone.

How Does Migration Work?

Migrations are stored as JavaScript files. Each migration defines a series of changes to be made to the database. When you run a migration, TypeORM will execute the defined changes.

Types of Migrations

There are two types of migrations:

• Up migrations: These migrations add or modify columns, tables, or other database objects.

• Down migrations: These migrations revert the changes made by up migrations.

Creating Migrations

To create a migration, you can use the typeorm migration:create command. This will generate a new JavaScript file in the migrations directory.

Example of a Migration

// Up migration
export async function up(queryRunner) {
  await queryRunner.addColumn("users", {
    name: "age",
    type: "int",
  });
}

// Down migration
export async function down(queryRunner) {
  await queryRunner.dropColumn("users", "age");
}

Real-World Applications

Migrations are useful for:

• Adding new features to your application

• Fixing bugs

• Updating to a new version of TypeORM

Tips

• Always write down migrations.

• Use descriptive names for your migrations.

• Test your migrations before running them on the production database.

---

Inserting Records

Inserting Records with TypeORM

What is Inserting Records?

Inserting records is the process of adding new data into a database. In TypeORM, you can insert records into your database using the insert() method.

How to Insert Records

To insert a new record, you first need to create an instance of the entity you want to insert. For example, let's say we have a User entity:

class User {
  id: number;
  name: string;
  email: string;
}

To insert a new user, we would create an instance of the User class and then use the insert() method:

const user = new User();
user.name = "John Doe";
user.email = "[e-mail]";

await connection.manager.insert(User, user);

Real-World Applications

Inserting records is a common operation in database applications. For example, you might use it to:

• Add new users to a user database

• Add new products to a product catalog

• Add new orders to an order database

Potential Applications

Here are some potential applications for inserting records with TypeORM:

• Building a user management system

• Creating a product catalog

• Developing an order processing system

Improved Code Example

Here is an improved version of the code example above:

import { getManager } from "typeorm";

async function insertUser() {
  const user = new User();
  user.name = "John Doe";
  user.email = "[e-mail]";

  const result = await getManager().insert(User, user);
  console.log("Inserted user with ID:", result.generatedMaps[0].id);
}

insertUser();

This code example uses the getManager() function to retrieve the database connection manager. It then uses the insert() method to insert the new user into the database. The result object contains the generated ID of the new user.

---

Integration with Jest

Integration with Jest

Jest is a popular testing framework for JavaScript and TypeScript, and it can be used with TypeORM to test your database interactions.

Getting Started

To use Jest with TypeORM, you will need to install the following dependencies:

npm install --save-dev @types/jest jest typeorm

Configuring Jest

You will need to add the following to your package.json file to configure Jest:

"jest": {
  "transform": {
    "^.+\\.tsx?$": "ts-jest"
  },
  "testRegex": "(/__tests__/.*|(\\.|/)(test|spec))\\.(jsx?|tsx?)$",
  "moduleFileExtensions": [
    "ts",
    "tsx",
    "js",
    "jsx",
    "json",
    "node"
  ]
}

Creating a Test Database

Before running any tests, you will need to create a test database. You can do this by running the following command:

npx typeorm migration:create -n test

Writing Tests

You can write tests for your TypeORM entities and repositories using the Jest API. Here is an example of a test for a user repository:

import { Repository } from "typeorm";
import { User } from "./User";

describe("UserRepository", () => {
  let userRepository: Repository<User>;

  beforeEach(() => {
    userRepository = getRepository(User);
  });

  it("should create a user", async () => {
    const user = new User();
    user.name = "John Doe";
    user.email = "[e-mail]";

    await userRepository.save(user);

    const foundUser = await userRepository.findOne(user.id);
    expect(foundUser).not.toBeNull();
    expect(foundUser.name).toBe("John Doe");
    expect(foundUser.email).toBe("[e-mail]");
  });
});

Running Tests

To run your tests, simply run the following command:

npm test

Real World Applications

Jest can be used to test any part of your TypeORM application, including:

• Entities

• Repositories

• Services

• Controllers

By writing tests, you can ensure that your code is working as expected and catch any potential bugs before they reach production.

Example

Here is a complete example of a Jest test for a TypeORM user repository:

import { Repository } from "typeorm";
import { User } from "./User";

describe("UserRepository", () => {
  let userRepository: Repository<User>;

  beforeEach(() => {
    userRepository = getRepository(User);
  });

  it("should create a user", async () => {
    const user = new User();
    user.name = "John Doe";
    user.email = "[e-mail]";

    await userRepository.save(user);

    const foundUser = await userRepository.findOne(user.id);
    expect(foundUser).not.toBeNull();
    expect(foundUser.name).toBe("John Doe");
    expect(foundUser.email).toBe("[e-mail]");
  });

  it("should update a user", async () => {
    const user = new User();
    user.name = "John Doe";
    user.email = "[e-mail]";

    await userRepository.save(user);

    user.name = "Jane Doe";
    await userRepository.save(user);

    const foundUser = await userRepository.findOne(user.id);
    expect(foundUser).not.toBeNull();
    expect(foundUser.name).toBe("Jane Doe");
    expect(foundUser.email).toBe("[e-mail]");
  });

  it("should delete a user", async () => {
    const user = new User();
    user.name = "John Doe";
    user.email = "[e-mail]";

    await userRepository.save(user);

    await userRepository.delete(user.id);

    const foundUser = await userRepository.findOne(user.id);
    expect(foundUser).toBeNull();
  });
});

This test suite covers the basic CRUD operations for a user repository. You can add additional tests to cover more complex scenarios.

---

Integration with Vue.js

Integration with Vue.js

What is it?

TypeORM is a great option for working with databases in Node.js, and it can be used seamlessly with Vue.js, a popular frontend framework.

How it works:

1. Install TypeORM and Vue.js:

# Install TypeORM
npm install typeorm
# Install Vue.js
npm install vue

2. Create a Vue.js project:

vue create my-project

3. Set up database connection:

// src/main.js
import { createConnection } from "typeorm";

// Create a connection to the database
createConnection({
  type: "postgres",
  host: "localhost",
  port: 5432,
  username: "postgres",
  password: "mysecretpassword",
  database: "my_database",
});

4. Create models:

// src/models/User.ts
import { Entity, PrimaryGeneratedColumn, Column } from "typeorm";

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @Column()
  email: string;
}

5. Use models in Vue.js components:

<template>
  <h1>{{ user.name }}</h1>
</template>

<script>
import { ref } from "vue";
import { getManager } from "typeorm";
import User from "@/models/User";

export default {
  setup() {
    const user = ref(null);

    // Fetch user from the database
    getManager()
      .findOne(User, 1)
      .then((u) => (user.value = u));

    return { user };
  },
};
</script>

Potential Applications:

• CRUD operations (create, read, update, delete) on database records

• Data binding and reactive updates in Vue.js components

• Real-time data synchronization with the database

• Building data-driven web applications

---

Connection Options

Connection Options

1. TypeOrmModuleOptions

This is the main configuration object for TypeORM and can be used to define various connection parameters such as database type, username, and password.

import { TypeOrmModuleOptions } from '@nestjs/typeorm';

const config: TypeOrmModuleOptions = {
  type: 'mysql',
  host: 'localhost',
  port: 3306,
  username: 'root',
  password: 'password',
  database: 'my_database',
  entities: [User], // Array of entities to be loaded into the database
  synchronize: true, // Automatically synchronizes the database schema with the entities
};

2. ConnectionOptions

This object represents the connection parameters that are used to establish a connection to the database.

import { ConnectionOptions } from 'typeorm';

const connectionOptions: ConnectionOptions = {
  type: 'mysql',
  host: 'localhost',
  port: 3306,
  username: 'root',
  password: 'password',
  database: 'my_database',
};

3. Additional Options

In addition to the main connection options, TypeORM also supports a wide range of additional options that can be used to customize the behavior of the ORM.

const options: TypeOrmModuleOptions = {
  ...connectionOptions, // Spread the connection options
  logging: 'all', // Enable all logging statements
  dropSchema: true, // Drops the database schema on connection
  migrationsRun: true, // Runs the migrations on connection
};

Real-World Applications

TypeORM is a powerful ORM that can be used in a variety of real-world applications. Some common use cases include:

• Managing database connections in a centralized and consistent manner.

• Simplifying database operations by providing a high-level API.

• Enhancing database performance through features like caching and query optimization.

• Enforcing database integrity by using constraints and relationships.

• Facilitating database migrations to ensure a smooth transition between database versions.

---

One-to-One Relationships

One-to-One Relationships in TypeORM

One-to-one relationships allow you to associate two database entities where one entity can have a maximum of one related entity and vice versa.

Simplified Explanation

Imagine you have two tables, Person and Address. Each person can have only one address, and each address can belong to only one person. This is a one-to-one relationship.

Code Example

// Person entity
@Entity()
export class Person {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  // One-to-one relationship with Address entity
  @OneToOne(() => Address, { cascade: true })
  address: Address;
}

// Address entity
@Entity()
export class Address {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  street: string;

  @Column()
  city: string;
}

In this example, the Person and Address entities have a one-to-one relationship. Each property has a cascade value of true, which means that if you delete or update a Person, the related Address will also be deleted or updated.

Real-World Applications

One-to-one relationships are used in many real-world applications, such as:

• User and Profile: A user can have only one profile, and a profile can belong to only one user.

• Product and Image: A product can have only one main image, and an image can belong to only one product.

• Order and Invoice: An order can have only one invoice, and an invoice can belong to only one order.

---

Change Tracking

Change Tracking in Node.js TypeORM

What is Change Tracking?

Change tracking is a feature in TypeORM that allows you to keep track of changes made to entities in your database. This is useful for:

• Auditing changes to data

• Undoing changes if necessary

• Detecting concurrent updates

How Change Tracking Works

TypeORM uses a special column called _version to track changes. When you save an entity, TypeORM increments the _version column. If you try to save an entity with an outdated _version column, TypeORM will throw an error.

Enabling Change Tracking

To enable change tracking for an entity, add the following decorator to the entity class:

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @VersionColumn()
  version: number;
}

Retrieving Changes

To retrieve changes made to an entity, you can use the EntityManager.findChanges method. This method returns an array of ChangeSet objects, which represent the changes made to each entity.

const entityManager = getManager();
const changes = await entityManager.findChanges(User);

Undoing Changes

If you need to undo changes made to an entity, you can use the EntityManager.revert method. This method reverts the entity to its previous state.

await entityManager.revert(user);

Real-World Applications

Change tracking can be used in a variety of real-world applications, including:

• Auditing: Change tracking can be used to track who made changes to data and when. This information can be used for compliance purposes or to investigate security incidents.

• Concurrency control: Change tracking can be used to prevent concurrent updates to the same data. This can help to ensure data integrity.

• Data recovery: Change tracking can be used to recover data if it is accidentally deleted or corrupted.

Additional Resources

• Change Tracking in TypeORM

• ChangeSet API

---

Soft Deletes

Soft Deletes in TypeORM

Imagine a database of customers, where each customer has a first name, last name, and email address. Sometimes, a customer may cancel their account or no longer wish to be in the system. Instead of deleting the entire record, we can "soft delete" it, which marks it as inactive but still keeps all the data.

Implementing Soft Deletes

To enable soft deletes in TypeORM, we add a @Column() decorator to our entity class with the type: "boolean", default: false option. We also add a deletedAt property to store the date and time when the entity was soft deleted:

import { Entity, Column, PrimaryGeneratedColumn } from "typeorm";

@Entity()
export class Customer {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  firstName: string;

  @Column()
  lastName: string;

  @Column()
  email: string;

  @Column({ type: "boolean", default: false })
  isDeleted: boolean;

  @CreateDateColumn()
  createdAt: Date;

  @UpdateDateColumn()
  updatedAt: Date;

  @DeleteDateColumn()
  deletedAt: Date;
}

Retrieving Entities

By default, soft-deleted entities are not included in queries. To retrieve them, we can use the withDeleted() method:

const customers = await connection.getRepository(Customer).find({ withDeleted: true });

Deleting Entities

To soft delete an entity, we can set its isDeleted property to true and save it:

const customer = await connection.getRepository(Customer).findOne(1);
customer.isDeleted = true;
await connection.getRepository(Customer).save(customer);

Real-World Applications

Soft deletes are useful in situations where we need to keep records for historical or auditing purposes, even if they are no longer active. For example:

• E-commerce: Customers may delete their accounts, but we still need to keep their order history.

• Banking: Closed accounts may need to be kept for compliance reasons.

• Healthcare: Patient records must be retained for a certain period of time, even if the patient is no longer active.

---

Indexing

Indexing

Indexing is a way to make searching a database faster. It's like adding tags to items in a library. When you search for a book, you can look through the tags instead of having to read every single book.

Types of Indexes

There are two main types of indexes:

1. Primary index: This is a unique index that identifies each row in the database. It's like the barcode on a book.

2. Secondary index: This is an index that's used to search for rows based on specific columns. It's like the table of contents in a book.

Creating an Index

You can create an index using the @Index() decorator:

@Index("name")
export class User {
  id: number;
  name: string;
}

This will create a secondary index on the name column.

Benefits of Indexing

Indexing can greatly improve the performance of your database. It can:

• Speed up searches

• Reduce the amount of work that the database has to do

• Make it easier to find specific data

Real-World Applications

Indexing is used in many real-world applications, such as:

• E-commerce websites: To search for products by name, price, or category

• Social media platforms: To search for users by name, location, or interests

• Data analysis platforms: To search for trends and patterns in large datasets

Code Implementations

Here is a complete code implementation for creating an index:

import {Entity, PrimaryGeneratedColumn, Column, Index} from "typeorm";

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  @Index()
  name: string;
}

This will create a table with a primary index on the id column and a secondary index on the name column.

---

Debugging

Debugging in TypeORM

Error Handling

TypeORM provides a mechanism to handle errors thrown during database operations. By default, TypeORM will log errors to the console. You can customize this behavior by providing a custom error handler.

// custom error handler
const errorHandler = (error: Error) => {
  // do something with the error
};

// set custom error handler
TypeORM.setErrorHandler(errorHandler);

Logging

TypeORM also provides a logging mechanism to help you debug your code. By default, TypeORM will log SQL queries and their execution time to the console. You can customize this behavior by providing a custom logger.

// custom logger
const logger = {
  log: (message: string) => {
    // do something with the message
  },
};

// set custom logger
TypeORM.setLogger(logger);

Debugging Tools

TypeORM provides a number of debugging tools to help you troubleshoot your code. These tools include:

• Database Inspector: A web-based interface that allows you to inspect the database schema, data, and queries.

• Query Profiler: A tool that helps you identify slow queries and optimize your database performance.

• Explain Plan: A tool that shows you the execution plan for a given query.

Real-World Applications

Debugging techniques can be used in a variety of real-world scenarios, such as:

• Troubleshooting errors that occur during database operations.

• Optimizing database performance by identifying slow queries.

• Understanding the execution plan for a given query.

Conclusion

Debugging is an essential part of software development. TypeORM provides a number of features and tools to help you debug your code and identify any issues. By using these techniques, you can ensure that your database operations are running smoothly and efficiently.

---

Integration with GraphQL

Integration with GraphQL

What is GraphQL?

GraphQL is a query language that allows you to request specific data from a server in a flexible and structured way. It uses a schema to define the data that can be retrieved and how it is organized.

Integration with TypeORM

TypeORM provides built-in support for GraphQL integration, allowing you to easily create GraphQL resolvers and queries for your database entities.

Steps for Integration:

1. Install GraphQL packages:

npm install apollo-server-express graphql typeorm-graphql

2. Create a GraphQL schema:

This defines the structure and types of data that can be queried.

import { gql } from 'graphql';

const schema = gql`
  type User {
    id: Int!
    name: String
    age: Int
  }

  type Query {
    users: [User]
    user(id: Int!): User
  }
`;

3. Create GraphQL resolvers:

These functions retrieve data from your database entities and return it in the format specified by the GraphQL schema.

import { Injectable } from '@nestjs/common';
import { In, Repository } from 'typeorm';
import { User } from './user.entity';
import { InjectRepository } from '@nestjs/typeorm';

@Injectable()
export class UserResolver {
  constructor(
    @InjectRepository(User) private readonly userRepository: Repository<User>,
  ) {}

  async users(): Promise<User[]> {
    return this.userRepository.find();
  }

  async user(id: number): Promise<User> {
    return this.userRepository.findOneBy({ id });
  }
}

4. Create an Apollo Server:

This serves as the interface between your GraphQL schema and resolvers, and manages requests and responses.

import { ApolloServer, gql } from 'apollo-server-express';

const server = new ApolloServer({
  typeDefs: schema,
  resolvers: [UserResolver],
});

5. Integrate with Express.js:

This allows you to use GraphQL alongside your Express.js application.

import { ApolloServer } from 'apollo-server-express';
import { Express } from 'express';

export const applyGraphQL = (app: Express) => {
  const server = new ApolloServer({
    typeDefs: schema,
    resolvers: [UserResolver],
  });
  server.applyMiddleware({ app });
};

Real-World Applications:

• User Dashboard: Retrieve information about users from a database, such as their names, ages, and roles.

• Product Catalog: Query for product information, including categories, descriptions, and pricing.

• Social Media Feed: Fetch data from a social media platform, such as posts, comments, and likes.

• E-commerce Checkout: Retrieve product information and user data for checkout purposes, such as shipping and billing addresses.

• Data Visualization: Create interactive visualizations that query data from various sources in real-time.

---

Integration with Angular

TypeORM Integration with Angular

Setup

1. Install the TypeORM Angular package:

npm install @nestjs/typeorm

2. Add TypeOrmModule to your Angular module:

import { TypeOrmModule } from '@nestjs/typeorm';

@NgModule({
  imports: [
    TypeOrmModule.forRoot({
      type: 'mysql',
      host: 'localhost',
      port: 3306,
      username: 'username',
      password: 'password',
      database: 'database',
      entities: [ /* Your entities here */ ],
      synchronize: true, // Auto-sync database with models
      logging: true, // Log SQL queries
    }),
  ],
})
export class AppModule {}

Using TypeORM

Once TypeORM is set up, you can use it to:

• Create entities representing database tables.

• Create repositories to perform CRUD operations on entities.

• Inject repositories into your Angular components or services.

Example

Consider an Article entity:

@Entity()
export class Article {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  title: string;

  @Column()
  content: string;
}

To use this entity, create a repository:

import { Repository } from 'typeorm';
import { Article } from './article.entity';

@Injectable()
export class ArticleRepository {
  constructor(@InjectRepository(Article) private articleRepository: Repository<Article>) {}

  async create(article: Article): Promise<Article> {
    return await this.articleRepository.save(article);
  }

  async findAll(): Promise<Article[]> {
    return await this.articleRepository.find();
  }
}

Finally, inject the repository into a component:

@Component({ /* ... */ })
export class ArticleListComponent {
  constructor(private articleRepository: ArticleRepository) {}

  async ngOnInit(): void {
    this.articles = await this.articleRepository.findAll();
  }
}

Real-World Applications

• Managing large and complex data models

• Performing CRUD operations on entities from multiple data sources

• Building data-driven web applications

• Reducing development time by avoiding repetitive SQL queries

---

One-to-Many Relationships

One-to-Many Relationships in Node.js with TypeORM

What is a One-to-Many Relationship?

Think of it like a music playlist. Each song (row) in the playlist is a child, and the playlist itself (table) is the parent. One playlist can have many songs, but each song belongs to only one playlist.

Creating a One-to-Many Relationship

1. Define the Parent Entity:

@Entity()
export class Playlist {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

2. Define the Child Entity:

@Entity()
export class Song {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  title: string;

  @ManyToOne(() => Playlist, playlist => playlist.songs)
  playlist: Playlist;
}

• ManyToOne() decorator links the Song to the Playlist parent.

• playlist property specifies the parent entity.

Example

const playlist = new Playlist();
playlist.name = "My Awesome Playlist";

const song1 = new Song();
song1.title = "Song 1";
song1.playlist = playlist;

const song2 = new Song();
song2.title = "Song 2";
song2.playlist = playlist;

Potential Applications

• Products and categories in an e-commerce website

• Employees and departments in a company

• Posts and comments on a social media platform

Advantages of One-to-Many Relationships

• Data consistency: Ensures that child records are always associated with a valid parent.

• Ease of retrieval: Allows easy access to all child records for a given parent.

• Improved performance: Can improve performance by storing related data in a single parent table.

---

Best Practices

TypeORM Best Practices

1. Use Lazy Loading to Improve Performance

Explanation: Lazy loading means the data for an entity is only loaded when it's requested. This can improve performance by reducing the amount of data transferred over the network.

Example:

// Get only the user's ID and name
const user = await userRepository.findOne({ id: 1 }, { select: ['id', 'name'] });

// Later, inside a loop or when needed, load the user's posts
const posts = await user.posts;

2. Prefer Eager Loading for Data You Need Immediately

Explanation: Eager loading means the data for an entity is loaded immediately. This is useful when you need the data right away.

Example:

// Get the user with all their posts
const user = await userRepository.findOne({ id: 1 }, { relations: ['posts'] });

3. Avoid N+1 Queries with Proper Entity Relations

Explanation: N+1 queries occur when you make multiple database calls to fetch related data. TypeORM can automatically join tables to reduce the number of queries.

Example:

// Get users with their posts in one query using the @OneToMany decorator
@Entity()
class User {
  @OneToMany(() => Post, post => post.user)
  posts;
}

// Query to get all users with their posts
const users = await userRepository.find();

4. Use Repository Methods for Common Operations

Explanation: TypeORM provides repository methods that make it easy to perform common operations like saving, updating, and deleting data.

Example:

// Saving a new user
await userRepository.save(user);

// Updating an existing user
await userRepository.update({ id: 1 }, { name: 'New Name' });

// Deleting a user
await userRepository.delete({ id: 1 });

5. Use Transaction Manager for Data Consistency

Explanation: Transactions ensure that multiple operations are executed atomically or not at all. TypeORM's transaction manager helps maintain data integrity.

Example:

// Start a new transaction
const transaction = await connection.transaction();

try {
  // Perform some operations inside the transaction
  await userRepository.save(user1);
  await userRepository.save(user2);

  // Commit the transaction if everything went well
  await transaction.commit();
} catch (error) {
  // If any error occurs, rollback the transaction
  await transaction.rollback();
}

6. Use Column Types Wisely for Optimal Storage

Explanation: Properly selecting column types can optimize storage and performance. TypeORM supports a wide range of data types.

Example:

// Use a string column for short text
@Column({ type: 'varchar', length: 255 })
name;

// Use an integer column for numbers
@Column({ type: 'int' })
age;

// Use a boolean column for true/false values
@Column({ type: 'boolean' })
isActive;

7. Consider Using UUIDs or Incremental IDs for Primary Keys

Explanation: UUIDs (Universally Unique Identifiers) and incremental IDs are commonly used for primary keys. UUIDs are randomly generated and unique, while incremental IDs are generated sequentially.

Example:

// Using a UUID primary key
@PrimaryColumn({ type: 'uuid' })
id;

// Using an incremental primary key
@PrimaryGeneratedColumn()
id;

8. Optimize Queries and Indexes for Fast Data Retrieval

Explanation: Proper query optimization and indexing can significantly improve performance. TypeORM provides helper functions for creating indexes.

Example:

// Create an index on the 'name' column
await connection.queryRunner.createIndex('user', 'name');

// Use an indexed query
const users = await userRepository.find({ order: { name: 'ASC' } });

9. Use Custom Repositories for Complex Queries and Logic

Explanation: Custom repositories allow you to define custom methods for complex queries or business logic. They extend the default repository functionality.

Example:

// Define a custom repository
import { EntityRepository } from 'typeorm';
@EntityRepository(User)
export class UserRepository extends Repository<User> {
  async findSpecialUsers() {
    return this.createQueryBuilder('user')
      .where('user.age > 30')
      .andWhere('user.isActive = true')
      .getMany();
  }
}

10. Handle Data Validation and Error Handling Properly

Explanation: Data validation and error handling are essential for maintaining data quality and handling exceptions. TypeORM provides validation decorators and error handling functions.

Example:

// Use validation decorators to validate data
@Column({ unique: true })
email;

// Handle errors using a try-catch block
try {
  await userRepository.save(user);
} catch (error) {
  // Log the error and handle it appropriately
  console.error(error);
}

---

Testing

Testing in Node.js TypeORM

Introduction: TypeORM is an ORM (Object-Relational Mapping) library for Node.js that makes it easier to work with databases. Testing is an important part of software development, and TypeORM provides tools to help you test your code.

Topics:

1. Unit Testing:

• Unit testing tests individual functions or methods in isolation.

• Example: Creating a test case to assert that a function returns the correct value for a given input.

2. Integration Testing:

• Integration testing tests how your code interacts with other parts of your application.

• Example: Creating a test that verifies the functionality of a repository that interacts with your database.

3. Functional Testing:

• Functional testing tests the overall functionality of your application.

• Example: Creating a test that simulates user actions and verifies the expected behavior of your application.

Simplified Explanation:

1. Unit Testing (like a puzzle piece):

• Tests small pieces of your code to make sure they work as expected.

• Like testing if a puzzle piece fits perfectly into its spot.

2. Integration Testing (like a car engine):

• Tests how different parts of your code work together.

• Like testing if all the parts of a car engine work together smoothly.

3. Functional Testing (like a finished car):

• Tests your entire application as a whole.

• Like testing if a car can drive safely and comfortably.

Code Implementations:

Unit Testing Example:

import { expect } from 'chai';
import { Repository } from 'typeorm';

describe('UserRepository', () => {
  let userRepository: Repository<User>;

  before(() => {
    // Setup test environment
  });

  it('should find by id', async () => {
    const user = await userRepository.findOne(1);
    expect(user.name).to.equal('John Doe');
  });
});

Integration Testing Example:

import { expect } from 'chai';
import { UserService } from './user.service';
import { Repository } from 'typeorm';

describe('UserService', () => {
  let userService: UserService;
  let userRepository: Repository<User>;

  before(() => {
    // Setup test environment
  });

  it('should create a user', async () => {
    const user = await userService.create({ name: 'John Doe' });
    expect(user.name).to.equal('John Doe');
  });
});

Functional Testing Example:

import { expect } from 'chai';
import * as request from 'supertest';
import { app } from '../server';

describe('User API', () => {
  it('should create a user', async () => {
    const response = await request(app).post('/api/users').send({ name: 'John Doe' });
    expect(response.status).to.equal(201);
    expect(response.body.name).to.equal('John Doe');
  });
});

Real World Applications:

Unit Testing:

• Ensure individual components of your application are working as intended.

• Helps prevent bugs and inconsistencies.

Integration Testing:

• Verify that different parts of your application work together seamlessly.

• Reduces the risk of unexpected errors in production.

Functional Testing:

• Test the functionality of your application from the user's perspective.

• Increases confidence in the overall quality and reliability of your application.

---

Caching

Caching

Caching is like having a fast-access memory zone that stores often-needed data. This way, instead of fetching data every time, we can grab it from the cache, which is much faster.

First-Level Cache (Connection-based)

Imagine the first-level cache as a bookshelf in your room. It's close at hand, so you can quickly grab your favorite book. Similarly, ORM manages its own cache per database connection. This cache stores recently fetched entities and can be accessed using entityCache property.

Second-Level Cache

The second-level cache acts like a shared bookshelf in your house, accessible from different rooms. It's a centralized cache shared among multiple connections. This is useful when you have multiple database connections and want to share data between them.

Custom Caching

Sometimes the built-in caching might not be enough. For such cases, TypeORM allows you to implement your own custom caching strategy. You can define how entities are stored and retrieved from the cache.

Real-World Applications

• Caching frequently accessed data like user preferences or product catalogs to improve website performance.

• Reducing database load by storing read-only data in the cache.

• Sharing real-time data between multiple services or processes using a second-level cache.

Code Examples

First-Level Cache:

// Get entity from the cache
const cachedEntity = connection.entityCache.get(EntityType, id);

// If entity is not in cache, fetch it from database
if (!cachedEntity) {
  const entity = await connection.getRepository(EntityType).findOne(id);
  connection.entityCache.save(entity);
}

Second-Level Cache:

// Configure second-level caching
connection.options.cache = {
  type: 'redis', // Use Redis as cache provider
  options: {
    host: 'localhost', // Redis host
    port: 6379, // Redis port
  },
};

// Get entity from the cache
const cachedEntity = connection.manager.secondLevelCache.get(EntityType, id);

// If entity is not in cache, fetch it from database
if (!cachedEntity) {
  const entity = await connection.getRepository(EntityType).findOne(id);
  connection.manager.secondLevelCache.save(entity);
}

Custom Caching:

// Define a custom caching strategy
@Entity()
class MyEntity {

  @Column()
  name: string;

  @Column()
  description: string;
}

@Injectable()
export class MyCustomCacheService {

  async save(entity: MyEntity): Promise<void> {
    // Implement custom caching logic
  }

  async get(id: number): Promise<MyEntity> {
    // Implement custom caching logic
  }
}

// Register the custom cache service
TypeORM.registerCache(MyCustomCacheService);

---

Inheritance Table Per Class

Inheritance: Table Per Class

In this inheritance strategy, each class in the hierarchy has its own database table. This approach is the simplest to implement and understand, but it can lead to data duplication if there are many common columns between the tables.

Schema

CREATE TABLE Person (
  id INT NOT NULL AUTO_INCREMENT,
  name VARCHAR(255) NOT NULL,
  PRIMARY KEY (id)
);

CREATE TABLE Student (
  id INT NOT NULL AUTO_INCREMENT,
  school VARCHAR(255) NOT NULL,
  PRIMARY KEY (id),
  FOREIGN KEY (id) REFERENCES Person(id)
);

CREATE TABLE Employee (
  id INT NOT NULL AUTO_INCREMENT,
  salary INT NOT NULL,
  PRIMARY KEY (id),
  FOREIGN KEY (id) REFERENCES Person(id)
);

Example

@Entity()
class Person {

  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

@Entity()
class Student extends Person {

  @Column()
  school: string;
}

@Entity()
class Employee extends Person {

  @Column()
  salary: number;
}

Advantages

• Simple to implement and understand.

• No data duplication if there are no common columns between the tables.

Disadvantages

• Data duplication if there are many common columns between the tables.

• Can be inefficient for queries that need to join multiple tables.

Real-World Applications

• A company with a hierarchy of employees, where each type of employee has different attributes.

• A school with a hierarchy of students, where each type of student has different attributes.

---

Entity Configuration

Entity Configuration in TypeORM

Imagine you have a database with tables for users and their orders. In TypeORM, each table is represented as an entity. To configure how TypeORM interacts with an entity, you use decorators. Let's explore these decorators:

@Entity()

This decorator marks a class as an entity. It's like saying, "Hey TypeORM, this class represents a table in the database."

@Column()

Use this decorator for each property in your entity class that maps to a column in the database. It specifies the column's name, type, and other options.

@PrimaryColumn()

This decorator marks a specific property as the primary key for the table. The primary key is used to uniquely identify each row in the table.

@OneToMany()

This decorator defines a one-to-many relationship between two entities. For example, if a user can have many orders, you'd use @OneToMany() to specify that the "orders" property in the "User" entity represents a collection of related "Order" entities.

@JoinColumn()

This decorator is used in conjunction with @OneToMany() to specify the foreign key column that joins the related entities. In the user-order example, the @JoinColumn() would define which column in the "Order" table links it to the "User" table.

Example:

Here's a simplified code example that demonstrates these decorators:

import { Entity, Column, PrimaryColumn, OneToMany, JoinColumn } from "typeorm";

@Entity()
export class User {
  @PrimaryColumn()
  id: number;

  @Column()
  name: string;

  @OneToMany(() => Order, order => order.user) // Defines the one-to-many relationship with the Order entity
  @JoinColumn() // Specifies the foreign key column in the Order table
  orders: Order[]; // Array of Order entities related to this User
}

@Entity()
export class Order {
  @PrimaryColumn()
  id: number;

  @Column()
  product: string;

  @Column()
  quantity: number;

  @JoinColumn() // Specifies the foreign key column in the Order table
  user: User; // User entity related to this Order
}

Real-World Applications:

• User Authentication: The '@Column()' decorator can be used to configure password hashing and other security features for user accounts.

• E-commerce: The '@OneToMany()' decorator can be used to represent the relationship between a product and its reviews or between a customer and their purchase history.

• Data Analytics: The '@PrimaryColumn()' decorator can be used to create unique identifiers for data points, making it easier to track and analyze data.

---

Integration with JavaScript

Introduction to JavaScript Integration with TypeORM

TypeORM is an ORM (Object-Relational Mapping) library that allows you to interact with databases using JavaScript. It simplifies the process of connecting to, querying, and manipulating data in relational databases.

Key Benefits of JavaScript Integration

• Simplified database operations: TypeORM provides an intuitive API that makes working with databases as easy as working with objects in JavaScript.

• Type safety: TypeORM enforces type checking, ensuring that you handle data correctly and avoid errors.

• Improved performance: TypeORM optimizes queries and uses caching to enhance application performance.

How to Integrate TypeORM with JavaScript

1. Installation

• Install TypeORM using NPM: npm install typeorm

2. Database Configuration

• Define your database connection details in ormconfig.json file:

{
  "type": "postgres",
  "host": "localhost",
  "port": 5432,
  "username": "postgres",
  "password": "mypassword",
  "database": "mydatabase"  
}

3. Entity Definition

• Create classes representing your database tables (e.g., User):

import { Entity, PrimaryGeneratedColumn, Column, Index } from "typeorm";

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  @Index()
  username: string;

  @Column()
  password: string;
}

4. Repository and Querying

• Use getRepository() method to interact with your entities:

// Get the user repository
const userRepository = connection.getRepository(User);

// Find all users
const users = await userRepository.find();

Real World Applications

• User management systems: TypeORM can be used to store and manage user accounts in a database, enabling user registration, login, and account management.

• E-commerce applications: TypeORM can be leveraged to handle product inventory, order processing, and customer information in online stores.

• Social media platforms: TypeORM can facilitate user profiles, posts, comments, and other features in social media applications.

Example Implementation

User Registration:

// Create a new user record
const newUser = new User();
newUser.username = "johndoe";
newUser.password = "mypassword";

// Save the user to the database
await userRepository.save(newUser);

User Login:

// Find the user by username
const user = await userRepository.findOne({ username: "johndoe" });

// Compare the entered password with the stored password
if (user && user.password === "mypassword") {
  // Login successful
}

---

Table Inheritance

Table Inheritance

Imagine you have a school with students and teachers. Each individual has a common attribute: a name. But students and teachers have unique attributes as well. Students have a grade, while teachers have a subject they teach.

In a database, you can model this relationship using table inheritance. This means creating a parent table (Person) with the shared attributes, and child tables (Student and Teacher) with the unique attributes.

Types of Table Inheritance

There are three main types of table inheritance:

1. Single Table Inheritance (STI): All data is stored in a single table. The Person table would have a column for type (student or teacher), and then separate columns for grade and subject.

2. Joined Table Inheritance (JTI): The shared data is stored in the parent table, while the unique data is stored in separate child tables. The Person table would contain the name. The Student table would have a foreign key to Person and contain grade. The Teacher table would also have a foreign key to Person and contain subject.

3. Table Per Class (TPC): Each class has its own separate table. The Person table would be empty, and there would be separate Student and Teacher tables with all their attributes.

Code Examples

STI:

@Entity()
class Person {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @Column({ type: "enum", enum: ["student", "teacher"] })
  type: string;
}

JTI:

@Entity()
class Person {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

@Entity()
class Student extends Person {
  @Column()
  grade: number;
}

@Entity()
class Teacher extends Person {
  @Column()
  subject: string;
}

TPC:

@Entity()
class Student {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @Column()
  grade: number;
}

@Entity()
class Teacher {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @Column()
  subject: string;
}

Real-World Applications

Table inheritance allows you to model complex relationships in a database. For example:

• E-commerce: You can create a Product table with shared attributes like name and price. Then, you can create child tables for specific product types, such as Book (with attributes like pages and ISBN) and Electronic (with attributes like wattage and voltage).

• Social Network: You can create a User table with common attributes like name and location. Then, you can create child tables for different user types, such as Employee (with attributes like job title and department) and Customer (with attributes like purchase history).

---

Error Handling

Error Handling in TypeORM

1. Custom Error Handling

• Create a custom error class that extends Error.

• Use throw to raise the custom error.

• Catch the error using try-catch blocks or async-await.

Code:

// Create a custom error class
class MyCustomError extends Error {
  constructor() {
    super('Custom error message');
  }
}

// Raise the custom error
throw new MyCustomError();

// Catch the error
try {
  // Code that may throw an error
} catch (err) {
  if (err instanceof MyCustomError) {
    // Handle the custom error
  } else {
    // Handle other errors
  }
}

2. Error Codes

• TypeORM provides defined error codes for specific errors.

• Use code property of the error object to get the error code.

Code:

// Get the error code
const code = err.code;

// Check for specific error codes
switch (code) {
  case 'ER_DUP_ENTRY':
    // Handle duplicate entry error
    break;
  case 'ER_NO_SUCH_TABLE':
    // Handle table not found error
    break;
  default:
    // Handle other errors
}

3. Entity Validation Errors

• TypeORM provides validation for entities.

• When entity validation fails, an array of validation errors is returned.

• Access the validation errors using validationErrors property of the error object.

Code:

// Get the validation errors
const validationErrors = err.validationErrors;

// Iterate over the validation errors
validationErrors.forEach((error) => {
  console.log(error.property, error.constraints);
});

4. Query Builder Errors

• Errors can occur during query execution using Query Builder.

• Use catch or async-await to catch Query Builder errors.

Code:

// Using try-catch
try {
  await queryBuilder.execute();
} catch (err) {
  // Handle the Query Builder error
}

// Using async-await
async function executeQuery() {
  try {
    await queryBuilder.execute();
  } catch (err) {
    // Handle the Query Builder error
  }
}

Real-World Applications:

• Custom Error Handling: Creating custom errors allows for more specific and informative error messages, making debugging easier.

• Error Codes: Using error codes enables developers to handle different types of errors in a structured and consistent manner.

• Entity Validation Errors: Validation errors help ensure that data stored in the database meets defined constraints.

• Query Builder Errors: Handling Query Builder errors allows developers to troubleshoot and fix issues when executing queries.

---

Integration with NestJS

Integration with NestJS

NestJS is a popular framework for building Node.js applications, and it can be integrated with TypeORM to make it easy to work with databases in your NestJS applications.

Installing TypeORM for NestJS

To install TypeORM for NestJS, you can use the following command:

npm install typeorm @nestjs/typeorm

Creating a TypeORM Module

Once you have TypeORM installed, you will need to create a TypeORM module. This module will provide the configuration for your TypeORM connection.

import { Module } from '@nestjs/common';
import { TypeOrmModule } from '@nestjs/typeorm';
import { User } from './user.entity';

@Module({
  imports: [
    TypeOrmModule.forRoot({
      type: 'postgres',
      host: 'localhost',
      port: 5432,
      username: 'postgres',
      password: 'secret',
      database: 'mydb',
      entities: [User],
    }),
  ],
})
export class TypeOrmModule {}

The TypeOrmModule.forRoot() method takes a configuration object as an argument. This object can be used to specify the type of database you are using, the host, port, username, password, and database name. You can also specify the entities that you want to use with TypeORM.

Using TypeORM in Your NestJS Controllers and Services

Once you have created a TypeORM module, you can use TypeORM in your NestJS controllers and services. To do this, you can use the @InjectRepository() decorator.

import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './user.entity';

@Injectable()
export class UserService {
  constructor(
    @InjectRepository(User)
    private readonly userRepository: Repository<User>,
  ) {}

  async findAll(): Promise<User[]> {
    return await this.userRepository.find();
  }
}

The @InjectRepository() decorator injects the repository for a given entity into your controller or service. This allows you to use TypeORM's methods to perform CRUD operations on your database.

Potential Applications

TypeORM can be used in a variety of real-world applications, such as:

• Building web applications that interact with databases

• Creating command-line tools that manage data

• Developing mobile applications that need to store data locally

Conclusion

TypeORM is a powerful ORM that can be integrated with NestJS to make it easy to work with databases in your NestJS applications. By following the steps outlined in this guide, you can get started with TypeORM and start using it to build powerful and scalable applications.

---

Performance Optimization

1. Use Indexed Columns

• What it means: When you search for data in a database table, the database needs to scan through all the rows to find the ones you want. If you create an index on a column, the database can jump directly to the rows that match your search criteria, making the search much faster.

• Simplified example: Imagine you have a library of books. If you want to find a book by its title, you could start by looking through every book in the library. But if you have an index that lists all the books by title, you can just flip to the right page and find the book you want much faster.

// Create an index on the `title` column of the `books` table
await connection.query(`CREATE INDEX title_index ON books (title)`);

2. Use Query Caching

• What it means: Query caching stores the results of frequently executed queries in memory, so that the database doesn't have to re-execute them every time. This can significantly improve the performance of your application, especially if you have many users running the same queries over and over again.

• Simplified example: Imagine you have a website that shows the latest news articles. Every time a user visits the site, the server needs to query the database to get the latest articles. With query caching, the server can simply read the articles from memory, which is much faster than re-executing the query every time.

// Enable query caching
await connection.query(`SET GLOBAL query_cache_type = ON`);

3. Avoid N+1 Queries

• What it means: N+1 queries is a situation where your application executes multiple queries to retrieve data that could have been retrieved in a single query. This can significantly slow down your application, especially if the queries are complex or involve large amounts of data.

• Simplified example: Imagine you have a table of users and a table of orders. If you want to get all the orders for a specific user, you might write a query like this:

// N+1 queries
const user = await connection.findOne(User, { id: 1 });
const orders = await connection.find(Order, { userId: user.id });

But this query requires two database round trips, one to get the user and another to get the orders. To avoid N+1 queries, you can use eager loading to retrieve the orders along with the user in a single query:

// No N+1 queries
const user = await connection.findOne(User, { id: 1 }, { eager: true });

4. Use Bulk Inserts and Updates

• What it means: Bulk inserts and updates allow you to insert or update multiple rows in a database table in a single query. This can be much more efficient than executing multiple individual queries, especially if you have a large amount of data to insert or update.

• Simplified example: Imagine you have a table of products and you want to update the prices of all the products in a certain category. With individual queries, you would need to execute a separate query for each product:

// Individual queries
const products = await connection.find(Product, { categoryId: 1 });
for (const product of products) {
  product.price = product.price * 1.1;
  await connection.save(product);
}

But with bulk updates, you can update all the products in a single query:

// Bulk updates
await connection.update(Product, { categoryId: 1 }, { price: () => "price * 1.1" });

5. Use Transactions

• What it means: Transactions allow you to group multiple database operations together into a single atomic unit. This means that either all of the operations in the transaction are executed successfully, or none of them are executed. Transactions are useful for ensuring the integrity of your data, especially when you are performing multiple operations that depend on each other.

• Simplified example: Imagine you are transferring money from one bank account to another. You need to decrement the balance of the first account and increment the balance of the second account. If you don't use a transaction, it is possible that one of the operations will fail, leaving your data in an inconsistent state. With a transaction, you can ensure that either both operations succeed or both operations fail.

// Transaction
await connection.transaction(async (transactionalEntityManager) => {
  const [fromAccount, toAccount] = await Promise.all([
    transactionalEntityManager.findOne(Account, { id: fromAccountId }),
    transactionalEntityManager.findOne(Account, { id: toAccountId }),
  ]);
  fromAccount.balance -= amount;
  toAccount.balance += amount;
  await transactionalEntityManager.save(fromAccount);
  await transactionalEntityManager.save(toAccount);
});

6. Use Read Replicas

• What it means: Read replicas are additional database servers that are synchronized with the primary database server. This allows you to distribute read traffic across multiple servers, which can improve the performance of your application, especially if you have a high volume of read traffic.

• Simplified example: Imagine you have a website that serves millions of users. If all of the users are reading data from the same database server, the server can become overloaded and slow down. With read replicas, you can distribute the read traffic across multiple servers, so that each server has to handle a smaller amount of traffic.

// Configure read replicas
const connection = await createConnection({
  type: 'mysql',
  host: 'primary-host',
  port: 3306,
  username: 'root',
  password: 'password',
  database: 'database_name',
  replication: {
    read: [{
      host: 'replica-host-1',
      port: 3306,
      username: 'root',
      password: 'password',
    }, {
      host: 'replica-host-2',
      port: 3306,
      username: 'root',
      password: 'password',
    }],
  },
});

Real-world applications:

• Indexed columns: Used in e-commerce websites to quickly search for products by name, category, or other attributes.

• Query caching: Used in content management systems to speed up the display of frequently accessed pages.

• Bulk inserts and updates: Used in data migration scenarios to quickly move large volumes of data between databases.

• Transactions: Used in online banking applications to ensure the integrity of financial transactions.

• Read replicas: Used in high-traffic web applications to handle the load of read traffic and improve performance.

---

Repository API

Repository API

Imagine your database as a library, where each table is a bookshelf and each row is a book. The Repository API is like a librarian who helps you manage and access the books (rows) in your library (database).

find()

This method is like asking the librarian to find all the books in a specific bookshelf (table). It returns an array of all the rows in the table.

const users = await repository.find();

findOne()

This method is like asking the librarian to find a specific book based on its title or ISBN number (row's primary key). It returns a single row from the table.

const user = await repository.findOne(1); // where 1 is the primary key of the user

save()

This method is like adding a new book to the library (inserting a row) or updating an existing book (updating a row). It returns the saved row.

const newUser = repository.create({ name: 'John' });
await repository.save(newUser);

update()

This method is like updating an existing book in the library (updating a row). It returns the updated row.

await repository.update({ id: 1 }, { name: 'John Doe' });

delete()

This method is like removing a book from the library (deleting a row). It returns a Promise that resolves to undefined.

await repository.delete(1); // where 1 is the primary key of the user

createQueryBuilder()

This method returns a powerful tool that allows you to build complex SQL queries using a chainable API. It's like having a personal SQL translator who helps you write custom queries tailored to your specific needs.

const queryBuilder = repository.createQueryBuilder('user');
queryBuilder
  .where('user.name like :name', { name: '%John%' })
  .orderBy('user.id', 'DESC')
  .limit(10);

const users = await queryBuilder.getMany();

Real-World Applications

• e-commerce: Managing user accounts, product inventory, and order information.

• social media: Storing user profiles, posts, and interactions.

• healthcare: Keeping track of patient records, appointments, and prescriptions.

• finance: Managing financial transactions, accounts, and investments.

---

Repository

Repository

A Repository is like a container for entities of a specific type. It provides methods to manage those entities, such as adding, updating, fetching, and deleting them.

Functions of a Repository

• insert(entity): Adds a new entity to the database.

• update(entity): Updates an existing entity in the database.

• delete(entity): Deletes an entity from the database.

• findOne(criteria): Fetches a single entity that matches the specified criteria.

• findMany(criteria): Fetches multiple entities that match the specified criteria.

• findOneOrFail(criteria): Same as findOne, but throws an error if no entity is found.

• findManyOrFail(criteria): Same as findMany, but throws an error if no entities are found.

Example Usage

// Import the repository
import { Repository } from "typeorm";

// Create a repository for the "User" entity
const userRepository = new Repository<User>();

// Insert a new user
const newUser = new User();
newUser.name = "John Doe";
userRepository.insert(newUser);

// Update an existing user
const existingUser = await userRepository.findOne(1);
existingUser.name = "Jane Doe";
userRepository.update(existingUser);

// Delete a user
await userRepository.delete(existingUser);

// Fetch all users
const users = await userRepository.findMany();

Real-World Applications

Repositories are essential for performing CRUD (Create, Read, Update, Delete) operations on entities in TypeORM. They are used in a wide variety of applications, such as:

• E-commerce websites: Managing products, customers, orders, etc.

• Social media platforms: Managing users, posts, comments, etc.

• Content management systems: Managing articles, pages, tags, etc.

---

Many-to-Many Relationships

Many-to-Many Relationships

Imagine you have two tables: students and courses. Each student can take multiple courses, and each course can be taken by multiple students. This is a many-to-many relationship.

How TypeORM Handles Many-to-Many Relationships

TypeORM creates a separate table to represent the relationship. In our example, it will create a table called student_course with two columns: student_id and course_id. Each row in this table represents a relationship between a student and a course.

Code Snippets

// Student entity
@Entity()
export class Student {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @ManyToMany(() => Course, { cascade: true }) // many-to-many relationship
  courses: Course[];
}

// Course entity
@Entity()
export class Course {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @ManyToMany(() => Student, { cascade: true }) // many-to-many relationship
  students: Student[];
}

Real-World Implementations

• Social Media: Users can follow multiple other users, and each user can have multiple followers.

• E-commerce: Products can belong to multiple categories, and each category can have multiple products.

• Education: Students can enroll in multiple courses, and each course can have multiple students.

Potential Applications

• Data Analysis: Analyze relationships between entities, such as the number of students taking a particular course.

• Recommendation Systems: Identify similar entities, such as courses that are frequently taken together by students.

• Fraud Detection: Identify suspicious transactions by detecting unusual patterns in relationships between entities.

---

Schema Syncing

Schema Syncing

Schema syncing is a process that helps you keep your database in sync with your TypeORM entity definitions. This ensures that your database has all the necessary tables, columns, and constraints to support your application.

Types of Schema Syncing

There are two main types of schema syncing:

• CREATE: This operation creates a new database schema based on your entity definitions.

• UPDATE: This operation updates an existing database schema to match your entity definitions.

How to Perform Schema Syncing

You can perform schema syncing through the following steps:

1. Establish a connection to your database:

   const connection = await createConnection();

2. Run the schema syncing command:

   • For CREATE:

     await connection.runMigrations();

   • For UPDATE:

     await connection.synchronize();

Potential Applications

Schema syncing has many potential applications, including:

• Database setup: Create a database schema from scratch based on your entity definitions.

• Database migrations: Update an existing database schema to match changes in your entity definitions.

• Database rollback: Revert a database schema to a previous state in case of errors.

Complete Code Example

// Establish connection to PostgreSQL database
const connection = await createConnection({
  type: "postgres",
  host: "localhost",
  port: 5432,
  username: "postgres",
  password: "mypassword",
  database: "mydb",
  entities: [User],
});

// Run schema syncing to create tables
await connection.runMigrations();

// Close connection after schema syncing
await connection.close();

In this example, we create a connection to a PostgreSQL database and run schema syncing to create a new database schema based on the User entity definition. Once the schema syncing is complete, we close the connection.

---

Data Seeding

Data Seeding with TypeORM

Data seeding is the process of populating a database with initial data. This can be useful for testing, development, or creating a base set of data for your application.

Seeding with TypeORM

TypeORM provides a simple and convenient way to seed your database. Here's how:

1. Create a seed file.

   • Create a new TypeScript file in your project, such as ./src/seeds/initial-data.seed.ts.

2. Import TypeORM.

   • Import the necessary TypeORM classes into your seed file:

   import { DataSource } from "typeorm";

3. Define your seed data.

   • Create an array or object containing the data you want to insert into the database. For example:

   const seedData = [
     { name: "John Doe", email: "[e-mail]" },
     { name: "Jane Doe", email: "[e-mail]" },
   ];

4. Insert the data into the database.

   • Use the DataSource class to connect to your database and insert the seed data:

   const dataSource = new DataSource({
     type: "postgres",
     host: "localhost",
     port: 5432,
     username: "postgres",
     password: "my-secret-password",
     database: "my-database",
   });
   
   await dataSource.initialize();
   
   const userRepository = dataSource.getRepository(User);
   await userRepository.save(seedData);
   
   await dataSource.destroy();

Real-World Applications

Data seeding has many potential applications in real-world scenarios, such as:

• Testing: Populating a database with test data can help you write more effective and reliable tests.

• Development: Seeding data can help you quickly set up a development environment with a consistent set of data.

• Base data: Creating a base set of data can be useful for initializing an application or creating a starting point for users.

Conclusion

Data seeding with TypeORM is a simple and convenient way to populate your database with initial data. Whether you're testing, developing, or just creating a base set of data, seed files can save you time and effort.

---

Updating Records

Updating Records in Node.js with TypeORM

What is TypeORM?

TypeORM is a Node.js library that simplifies database operations by automatically mapping your JavaScript classes to the database. This makes it easy to create, read, update, and delete records in the database.

Updating Records

To update a record in the database using TypeORM, follow these steps:

1. Get the record you want to update. You can use the findOne() or find() methods to get the record.

2. Make changes to the record. You can modify the properties of the record directly.

3. Save the changes. Use the save() method to save the updated record to the database.

Code Example

// Get the post you want to update
const post = await manager.findOne(Post, 1);

// Update the post's title
post.title = "New Title";

// Save the changes to the database
await manager.save(post);

Real-World Applications

• Edit user profiles: You can use TypeORM to update user profiles, such as name, email, and password.

• Update product inventory: You can use TypeORM to update the stock levels of products in an e-commerce system.

• Change the status of an order: You can use TypeORM to update the status of orders, such as from "Processing" to "Shipped".

Potential Improvements

• Use transactions: When updating multiple records, it's a good practice to use transactions to ensure that all updates are successful or none of them are.

• Handle optimistic locking: TypeORM supports optimistic locking to prevent concurrent updates from overwriting each other.

• Use query builders: Query builders can be used to construct more complex update queries.

Additional Notes

• TypeORM uses active record patterns, which means that you can directly modify the properties of a record object to update it.

• You can use the update() method to update multiple records at once, but this approach is not as flexible as using the save() method.

• TypeORM supports different database providers, including MySQL, PostgreSQL, and SQL Server.

---

Logging

Logging in TypeORM

Imagine you have an application that uses a database to store information. When you make changes to the database, it can be helpful to keep a record of those changes. This is where logging comes in. Logging allows you to record all the changes made to the database, including who made the changes and when.

Types of Logging

• Entity Logging: Logs all changes made to entities (objects) in the database. This can be useful for tracking who made changes to specific data and when.

• Query Logging: Logs the SQL queries executed against the database. This can be useful for debugging and performance analysis.

• Migration Logging: Logs the execution of database migrations. Migrations are changes to the database schema (structure).

How to Use Logging

To use logging in TypeORM, you need to create a Logger object. You can then use the Logger object to log messages of different types.

// Create a logger
const logger = new Logger();

// Log an entity change message
logger.logEntityChange("User", "Updated user with id 1");

// Log a query execution message
logger.logQuery("SELECT * FROM user WHERE id = 1");

Applications in the Real World

Logging is useful in many real-world applications:

• Debugging: Logging can help you track down errors in your application by showing you what queries were executed and what changes were made to entities.

• Security: Logging can help you track who made changes to sensitive data.

• Performance analysis: Logging can help you analyze the performance of your application by showing you how long queries take to execute.

• Compliance: Logging can help you comply with regulatory requirements that require you to keep track of changes to your database.

---

Database Schema Update Strategies

Database Schema Update Strategies in TypeORM

TypeORM provides several strategies for handling database schema updates. These strategies determine how changes made to your entities in code are reflected in the actual database schema.

1. Create Drop

• Definition: Drops the entire existing database schema and creates a new one based on the current entity definitions.

• When to use:

  • When starting a new project or making significant changes to the schema.

  • When you want to ensure that the database schema matches the entity definitions exactly, discarding any existing data.

• Code Example:

// dropDatabase: true drops the existing schema and creates a new one
@Entity({ dropDatabase: true })
export class User {
  // ...
}

2. Update

• Definition: Updates the existing database schema to match the current entity definitions. It adds new columns or tables and modifies existing ones if necessary.

• When to use:

  • When making minor changes to the schema, such as adding or removing columns.

  • When you want to preserve existing data and only adjust the schema.

• Code Example:

// update: true updates the existing schema
@Entity({ update: true })
export class User {
  // ...
}

3. Synchronize

• Definition: Synchronizes the database schema with the current entity definitions. It performs both CREATE and DROP operations to ensure that the schema exactly matches the entities.

• When to use:

  • When you want to make sure that the database schema is always up-to-date with the entity definitions, regardless of any existing data.

  • Useful for maintaining a consistent schema in development environments.

• Code Example:

await connection.synchronize();  // Synchronizes the schema

4. None

• Definition: Does not perform any schema updates. The database schema remains unchanged.

• When to use:

  • When you want to manually control schema updates outside of TypeORM.

  • When integrating TypeORM with existing databases that have already been defined.

• Code Example:

// Do not perform any schema updates
@Entity({ synchronize: false })
export class User {
  // ...
}

Real-World Applications

• Create Drop: Used when migrating to a new database or making significant structural changes.

• Update: Used for incremental schema changes, such as adding or removing columns. Preserves existing data.

• Synchronize: Ideal for development environments or when you want to enforce a consistent schema across different versions of your application.

• None: Useful for legacy systems or when you prefer to manage schema updates manually.

---

Installation and Setup

Installation and Setup

Prerequisites:

• Node.js (v14 or higher)

• npm (package manager)

Installing TypeORM:

npm install typeorm

Creating a TypeORM Configuration File:

touch ormconfig.json

Contents of ormconfig.json (MySQL example):

{
  "type": "mysql",
  "host": "localhost",
  "port": 3306,
  "username": "root",
  "password": "password",
  "database": "mydb",
  "synchronize": true
}

Explain the fields in ormconfig.json:

• type: Database type (e.g., "mysql", "postgres", "sqlite")

• host: Database server host

• port: Database server port

• username: Database username

• password: Database password

• database: Database name

• synchronize: Automatically synchronize the database schema with the TypeScript model classes

Establishing a Connection:

import { createConnection } from "typeorm";

createConnection().then(async connection => {
  // Query the database
  const users = await connection.query("SELECT * FROM users");

  // Close the connection
  await connection.close();
}).catch(error => {
  console.log(error);
});

Defining Model Classes:

Model classes represent entities in your database:

class User {
  id: number;
  name: string;
}

Real-World Applications:

• Building RESTful APIs: Creating database models and performing CRUD operations.

• Data persistence: Storing data in a structured and persistent manner.

• Data querying: Retrieving and filtering data based on specific criteria.

• Database migration management: Keeping your database schema up-to-date with code changes.

---

Inheritance Table Per Concrete

Table Per Concrete Inheritance

Imagine you have a building with different types of rooms: bedrooms, kitchens, bathrooms. Each room has its own unique features, but they all have some common things like walls, a floor, and a ceiling.

In programming, we can represent this hierarchy using inheritance. We create a base class called Room that defines the common features. Then, we create derived classes for each type of room: Bedroom, Kitchen, and Bathroom.

Table Per Concrete is a way to store these different types of rooms in a database. Instead of creating one table for all rooms, we create a separate table for each type of room. This makes it easier to store and retrieve the data for each type of room.

Example:

// Base class Room
@Entity()
class Room {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  type: string;

  @Column()
  size: number;
}

// Derived class Bedroom
@Entity()
class Bedroom extends Room {

  @Column()
  numBeds: number;
}

// Derived class Kitchen
@Entity()
class Kitchen extends Room {

  @Column()
  numSinks: number;
}

// Derived class Bathroom
@Entity()
class Bathroom extends Room {

  @Column()
  numToilets: number;
}

Real-World Application:

• Storing real estate listings: You can create a base class for all listings and then derived classes for specific types of listings (e.g., apartments, houses, condos).

• Managing a hospital: You can create a base class for patients and then derived classes for different types of patients (e.g., inpatients, outpatients, emergency patients).

• Tracking inventory: You can create a base class for all inventory items and then derived classes for different types of items (e.g., electronics, clothing, furniture).

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Reverting Migrations

Reverting Migrations

Imagine you have a database table with columns for name, age, and balance. You run a migration to add a new column for email. However, after testing you realize the email column is not needed. This is where reverting migrations comes into play.

Reversion:

• Reversion is the process of rolling back a migration, restoring the database to its previous state.

• This allows you to undo any changes made during a migration.

How to Revert Migrations:

To revert a migration, use the following steps:

1. Identify the Migration: Find the migration you want to revert from your migration history.

2. Run the Reversion: Use the typeorm-migration:revert command followed by the migration's name, e.g., typeorm-migration:revert MyMigration.

Example:

typeorm-migration:revert AddEmailColumn

Recovering Down:

• After reverting a migration, the database is not fully restored. You need to run the migrations down from the reverted migration to the initial migration.

• This ensures that all the changes made during the migrations are undone.

How to Run Migrations Down:

To run migrations down, use the following steps:

1. Find the Last Migration: Identify the last migration that was reverted.

2. Run the Command: Use the typeorm-migration:down command followed by the migration's name, e.g., typeorm-migration:down MyLastMigration.

Example:

typeorm-migration:down MyLastMigration

Applications:

• Mistakes: Reverting migrations allows you to correct errors or undo unwanted changes.

• Database Refactoring: If you need to reorganize or restructure your database, you can use migrations to make the changes and revert them if necessary.

• Testing: To test your migrations, you can run them and then revert them to reset the database to its original state.

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Security Considerations

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ERROR OCCURED Security Considerations

    Can you please simplify and explain  the given content from nodejs typeorm's Security Considerations topic?
    - explain each topic in detail and simplified manner (simplify in very plain english like explaining to a child).
    - retain code snippets or provide if you have better and improved versions or examples.
    - give real world complete code implementations and examples for each.
    - provide potential applications in real world for each.
    - ignore version changes, changelogs, contributions, extra unnecessary content.
    

    
    The response was blocked.

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Entity Relationships

Entity Relationships

In a database, entities represent real-world objects. Relationships define how entities are connected to each other.

Types of Relationships:

One-to-One:

• One entity can have only one related entity.

• Example: A student can have only one unique student ID.

One-to-Many:

• One entity can have many related entities.

• Example: A teacher can have many students.

Many-to-One:

• Many entities can have only one related entity.

• Example: Many students can have only one teacher.

Many-to-Many:

• Many entities can have many related entities.

• Example: Many students can take many courses.

Creating Relationships in TypeORM:

// One-to-One
@OneToOne(type => Student, student => student.id)
student: Student;

// One-to-Many
@OneToMany(type => Course, course => course.teacher)
courses: Course[];

// Many-to-One
@ManyToOne(type => Teacher, teacher => teacher.students)
teacher: Teacher;

// Many-to-Many
@ManyToMany(type => Course, course => course.students)
courses: Course[];

Real-World Applications:

One-to-One:

• Managing employee records, where each employee has a unique employee ID.

One-to-Many:

• Tracking a teacher's students, where each teacher can have multiple students.

Many-to-One:

• Identifying the author of a book, where each book has a single author.

Many-to-Many:

• Managing student enrollments in courses, where students can enroll in multiple courses and courses can have multiple students.

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Querying

Querying in TypeORM

Basic Queries

• findById(id): Retrieves an entity by its ID.

• findOne(), findOneOrFail(): Retrieves a single entity that satisfies the given conditions.

• find(), findAndCount(): Retrieves all entities that satisfy the given conditions.

// Import the User entity
import { User } from "./User";

// Create a repository for the User entity
const userRepository = getRepository(User);

// Find a user by their ID
const user = await userRepository.findById(1);

// Find a user by their username
const user = await userRepository.findOne({ where: { username: "johndoe" } });

// Find all users with the first name "John"
const users = await userRepository.find({ where: { firstName: "John" } });

Advanced Queries

• QueryBuilder: A powerful interface for building complex queries.

• raw(): Executes a raw SQL query and returns the results.

// Import the QueryBuilder
import { QueryBuilder } from "typeorm";

// Create a QueryBuilder for the User entity
const queryBuilder = userRepository.createQueryBuilder("user");

// Build a complex query
const queryBuilder
  .where("user.firstName = :firstName", { firstName: "John" })
  .andWhere("user.lastName = :lastName", { lastName: "Doe" });

// Execute the query
const users = await queryBuilder.getMany();

// Executes a raw SQL query
const results = await userRepository.raw("SELECT * FROM users WHERE firstName = ?", ["John"]);

Real-World Applications

• Data retrieval: Fetching data from the database for display or processing.

• Search functionality: Implementing search features by filtering or sorting data based on user input.

• Data analysis: Extracting insights from data by aggregating or grouping it.

• Database maintenance: Updating, deleting, or inserting records as needed.