# sequelize

***

**1. Basic Model Creation and Querying**

```js
const { Sequelize } = require('sequelize');
const sequelize = new Sequelize('database', 'username', 'password', {
  host: 'localhost',
  dialect: 'postgres',
});

const User = sequelize.define('User', {
  firstName: Sequelize.STRING,
  lastName: Sequelize.STRING,
});

User.create({ firstName: 'John', lastName: 'Doe' });
User.findAll().then(users => console.log(users));
```

**2. Model Validation and Error Handling**

```js
const User = sequelize.define('User', {
  firstName: {
    type: Sequelize.STRING,
    allowNull: false,
    validate: {
      notNull: { msg: 'First name is required' },
    },
  },
  lastName: Sequelize.STRING,
});

User.create({ lastName: 'Doe' })
  .then(user => console.log(user))
  .catch(err => console.error(err.errors));
```

**3. Model Associations - One-to-Many**

```js
const Post = sequelize.define('Post', {
  title: Sequelize.STRING,
});

const Comment = sequelize.define('Comment', {
  content: Sequelize.TEXT,
});

Post.hasMany(Comment, { foreignKey: 'postId' });
Comment.belongsTo(Post, { foreignKey: 'postId' });
```

**4. Model Associations - Many-to-Many**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
});

const Project = sequelize.define('Project', {
  name: Sequelize.STRING,
});

User.belongsToMany(Project, { through: 'UserProjects' });
Project.belongsToMany(User, { through: 'UserProjects' });
```

**5. Model Scopes**

```js
const Task = sequelize.define('Task', {
  title: Sequelize.STRING,
  status: Sequelize.STRING,
});

const urgentScope = { where: { status: 'urgent' } };
Task.scope(urgentScope).findAll();
```

**6. Model Hooks**

```js
const Task = sequelize.define('Task', {
  title: Sequelize.STRING,
});

Task.beforeCreate(task => {
  task.title = task.title.toUpperCase();
});
```

**7. Transactions**

```js
sequelize.transaction(async t => {
  await User.create({ ... }, { transaction: t });
  await Task.create({ ... }, { transaction: t });
});
```

**8. Bulk Insert**

```js
User.bulkCreate([
  { firstName: 'John', lastName: 'Doe' },
  { firstName: 'Jane', lastName: 'Doe' },
]);
```

**9. Model Query Builder**

```js
User.findAll({
  where: {
    firstName: 'John',
    [Sequelize.Op.or]: [{ lastName: 'Doe' }, { lastName: 'Smith' }],
  },
});
```

**10. Model Relationships - Eager Loading**

```js
User.findAll({
  include: [
    { model: Task, as: 'tasks' },
    { model: Project, as: 'projects' },
  ],
});
```

**11. Model Relationships - Lazy Loading**

```js
User.findByPk(1).then(user => user.getTasks());
```

**12. Model Instance Methods**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
  email: Sequelize.STRING,
});

User.prototype.greet = function() {
  console.log(`Hello, my name is ${this.name}!`);
};

User.findByPk(1).then(user => user.greet());
```

**13. Model Class Methods**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
  email: Sequelize.STRING,
});

User.findByName = name => User.findAll({ where: { name } });

User.findByName('John Doe').then(users => console.log(users));
```

**14. Model Indexes**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
  email: Sequelize.STRING,
}, {
  indexes: [
    { fields: ['name'] },
    { fields: ['email'] },
  ],
});
```

**15. Model Data Types (Advanced)**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
  email: {
    type: Sequelize.STRING,
    validate: { isEmail: true },
  },
  birthdate: Sequelize.DATE,
  age: {
    type: Sequelize.VIRTUAL,
    get() {
      const now = new Date();
      const birthdate = new Date(this.birthdate);
      return now.getFullYear() - birthdate.getFullYear();
    },
  },
});
```

**16. Custom Query Logging**

```js
const originalQuery = sequelize.query.bind(sequelize);
sequelize.query = (query, options) => {
  console.log(`[SQL Query] ${query}`);
  return originalQuery(query, options);
};
```

**17. Model Factory**

```js
const UserFactory = sequelize.define('User', {
  name: Sequelize.STRING,
});

UserFactory.build({ name: 'John Doe' }); // Creates a new User object without saving it to the database
UserFactory.create({ name: 'John Doe' }); // Creates a new User object and saves it to the database
```

**18. Custom Model Finders**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
});

User.addScope('active', { where: { status: 'active' } });
User.scope('active').findAll(); // Finds all active users
```

**19. Model Virtual Fields**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
  age: {
    type: Sequelize.VIRTUAL,
    get() {
      const now = new Date();
      const birthdate = new Date(this.birthdate);
      return now.getFullYear() - birthdate.getFullYear();
    },
  },
});
```

**20. Model Callbacks**

```js
const User = sequelize.define('User', {
  name: Sequelize.STRING,
});

User.addHook('afterCreate', user => {
  console.log(`User ${user.name} created!`);
});
```

**21. Custom Sequelize Error Handling**

```js
const originalErrorHandler = sequelize.getErrorHandler();
sequelize.setErrorHandler(err => {
  if (err.name === 'SequelizeUniqueConstraintError') {
    console.error('Unique constraint violation!');
  } else {
    originalErrorHandler(err);
  }
});
```

**22. Using Transactions with Declarative Models**

```js
const transaction = sequelize.transaction();
await User.create({ ... }, { transaction });
await Post.create({ ... }, { transaction });
await transaction.commit(); // Commits the transaction and saves the changes to the database
```

**23. Using Transactions with Sequelize Instances**

```js
const user = await User.findByPk(1);
await user.update({ ... }, { transaction });
await Post.create({ ... }, { transaction });
await transaction.commit();
```

**24. Using Sequelize Models in TypeScript**

```typescript
// User model definition
class User extends Model {
  id!: number;
  name!: string;
}

// Define the model in Sequelize
User.init({
  id: {
    type: Sequelize.INTEGER,
    primaryKey: true,
    autoIncrement: true,
  },
  name: {
    type: Sequelize.STRING,
    allowNull: false,
  },
}, {
  sequelize,
  modelName: 'User',
});
```

**25. Using Sequelize Relationships with TypeScript**

```typescript
// Define the User and Post models
class User extends Model {
  id!: number;
  name!: string;
}

class Post extends Model {
  id!: number;
  title!: string;
  userId!: number;
}

// Define the relationship between User and Post
User.hasMany(Post, {
  foreignKey: 'userId',
  as: 'posts',
});
Post.belongsTo(User, {
  foreignKey: 'userId',
  as: 'user',
});
```