How to query a Many- to -Many relation with TypeORM

Querying many-to-many relationships in TypeORM typically involves several steps: establishing entity associations, defining relationships, and executing queries to fetch related data. I will walk you through these steps in detail.

Defining Entities and Relationships

Assume we have two entities: User and Group, which have a many-to-many relationship. First, we need to define this relationship in each entity.

typescript
import { Entity, PrimaryGeneratedColumn, Column, ManyToMany, JoinTable } from 'typeorm';
import { Group } from './Group';

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

  @Column()
  name: string;

  @ManyToMany(() => Group, group => group.users)
  @JoinTable()
  groups: Group[];
}

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

  @Column()
  name: string;

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

In this example, the @ManyToMany decorator defines the many-to-many relationship between entities, while the @JoinTable decorator on the User entity specifies it as the owning side, creating the join table.

Executing Queries

The steps to query many-to-many relationships can be performed in several ways. Here are some methods to retrieve related data:

1. Using QueryBuilder

We can construct a query using createQueryBuilder, then employ leftJoinAndSelect to join and select the Group entity. This ensures related Group data is included in the results.

typescript
import { getRepository } from 'typeorm';
import { User } from './User';

async function findUsersWithGroups() {
  const userRepository = getRepository(User);
  const users = await userRepository.createQueryBuilder('user')
    .leftJoinAndSelect('user.groups', 'group')
    .getMany();

  return users;
}

2. Using the find Method

Here, we use the find method with the relations option to specify relationships to load. This automatically fetches associated groups for users.

typescript
import { getRepository } from 'typeorm';
import { User } from './User';

async function findUsersWithGroups() {
  const userRepository = getRepository(User);
  const users = await userRepository.find({ relations: ['groups'] });
  return users;
}

3. Using QueryBuilder with Specific Conditions

In this query, we join the Group entity and add a condition to filter for a specific group name.

typescript
import { getRepository } from 'typeorm';
import { User } from './User';

async function findUsersInSpecificGroup(groupName: string) {
  const userRepository = getRepository(User);
  const users = await userRepository.createQueryBuilder('user')
    .innerJoinAndSelect('user.groups', 'group')
    .where('group.name = :groupName', { groupName })
    .getMany();

  return users;
}

These are fundamental methods for querying many-to-many relationships in TypeORM. In practical applications, these approaches can be combined and extended to meet various data query requirements.