How to save many to many in nestjs

When developing with the NestJS framework, managing many-to-many relationships typically involves using ORM libraries such as TypeORM or Sequelize. Here, I'll use TypeORM as an example to demonstrate how to set up and manage many-to-many relationships in NestJS. Using a common example, such as the many-to-many relationship between User and Role, to illustrate the process.

Step 1: Creating Entities

First, we need to create two entities for User and Role, and define their many-to-many relationship within these entities.

typescript
// user.entity.ts
import { Entity, PrimaryGeneratedColumn, Column, ManyToMany, JoinTable } from 'typeorm';
import { Role } from './role.entity';

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

  @Column({ length: 100 })
  name: string;

  @ManyToMany(() => Role, role => role.users)
  @JoinTable()
  roles: Role[];
}

// role.entity.ts
import { Entity, PrimaryGeneratedColumn, Column, ManyToMany } from 'typeorm';
import { User } from './user.entity';

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

  @Column({ length: 100 })
  name: string;

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

In the above code, the @ManyToMany() decorator is used to establish the many-to-many relationship between the two entities. The @JoinTable() decorator is used to specify the join table for this relationship, which is typically defined in only one entity.

Step 2: Database Migration or Synchronization

Ensure that your database matches these new entities and relationships. If you are using TypeORM's auto-sync feature (not recommended for production environments), TypeORM will automatically create or modify database tables to match your entity definitions when the application starts.

Step 3: Creating or Updating Data

In the service or controller, you may need to write logic to create or update user and role data. For example, you might need to add a user and associate it with specific roles.

typescript
// user.service.ts
import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './entities/user.entity';
import { Role } from './entities/role.entity';

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

  async addUserWithRoles(userName: string, roleNames: string[]): Promise<User> {
    const user = new User();
    user.name = userName;
    user.roles = await this.roleRepository.find({
      where: {
        name: In(roleNames)
      }
    });

    return this.userRepository.save(user);
  }
}

In this example, the addUserWithRoles method first creates a new User object and finds the corresponding Role objects based on the input role names. Then, it assigns these role objects to the user's roles property and saves the user.

Step 4: Querying Data

Querying data involving many-to-many relationships is straightforward.

typescript
async getUsersWithRoles(): Promise<User[]> {
  return this.userRepository.find({ relations: ["roles"] });
}

In the above method, the relations option in the find method tells TypeORM to also fetch the associated roles when querying users.

Summary

In NestJS, managing many-to-many relationships involves defining the correct entity relationships, ensuring database tables are properly set up, and correctly handling these relationships in business logic. The steps above demonstrate how to use TypeORM in a NestJS project to manage many-to-many relationships. This structure not only helps maintain clean code but also makes data operations more intuitive and manageable.