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To implement a Many to Many relationship using NestJs Sequelize and Mysql2, you can follow these steps:

1. Define your models: Define the two models that you want to associate with a many-to-many relationship. For example, if you have two models - User and Event, and you want to associate many Users with many Events, define the models like this:

// User.model.ts
import { Column, Model, Table } from 'sequelize-typescript';

@Table
export class User extends Model<User> {
  @Column
  name: string;
}

// Event.model.ts
import { Column, Model, Table } from 'sequelize-typescript';

@Table
export class Event extends Model<Event> {
  @Column
  title: string;
}

1. Define your association table: Since Sequelize does not natively support many-to-many relationships, you need to define a third table - an association table - to hold the associations between the two models. For example, you can define an association table with columns userId and eventId like this:

// UserEvent.model.ts
import { Column, ForeignKey, Model, Table } from 'sequelize-typescript';
import { Event } from './event.model';
import { User } from './user.model';

@Table
export class UserEvent extends Model<UserEvent> {
  @ForeignKey(() => User)
  @Column
  userId: number;

  @ForeignKey(() => Event)
  @Column
  eventId: number;
}

1. Define the associations: In each model's definition, you need to define the association that it has with the other model through the association table. For example, in the User model, you can define the many-to-many association it has with the Event model like this:

// User.model.ts
import { BelongsToMany, Column, Model, Table } from 'sequelize-typescript';
import { Event } from './event.model';
import { UserEvent } from './user-event.model';

@Table
export class User extends Model<User> {
  @Column
  name: string;

  @BelongsToMany(() => Event, () => UserEvent)
  events: Event[];
}

Similarly, in the Event model, you can define the many-to-many association it has with the User model like this:

// Event.model.ts
import { BelongsToMany, Column, Model, Table } from 'sequelize-typescript';
import { Event } from './event.model';
import { UserEvent } from './user-event.model';

@Table
export class Event extends Model<Event> {
  @Column
  title: string;

  @BelongsToMany(() => User, () => UserEvent)
  users: User[];
}

1. Sync the models with the database: Finally, sync the models with the database using Sequelize's sync method, like this:

// app.module.ts
import { Module } from '@nestjs/common';
import { SequelizeModule } from '@nestjs/sequelize';
import { User } from './models/user.model';
import { Event } from './models/event.model';
import { UserEvent } from './models/user-event.model';

@Module({
  imports: [
    SequelizeModule.forRoot({
      dialect: 'mysql',
      host: 'localhost',
      port: 3306,
      username: 'root',
      password: 'password',
      database: 'testdb',
      models: [User, Event, UserEvent],
      autoLoadModels: true,
      synchronize: true
    })
  ]
})
export class AppModule {}

This will create the necessary tables in the database, with the association table UserEvent holding the associations between the User and Event models. You can then use Sequelize's methods to interact with the models and their associations, for example:

const user = await User.findOne({ where: { name: 'John' } });
const event = await Event.findOne({ where: { title: 'Event1' } });

// Associate the user with the event
await user.$add('event', event);

// Get all events associated with the user
const events = await user.$get('events');