How can I define many to many columns with NestJS and TypeORM?

When defining many-to-many relationships using NestJS and TypeORM, you first need to define two entity classes and establish the association between them. The following is a specific example illustrating how to define such a many-to-many relationship.

Entity Definition

Assume we have two entities: Student and Course, where a student can enroll in multiple courses, and a course can be selected by multiple students.

1. Student Entity

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

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

  @Column()
  name: string;

  @ManyToMany(() => Course, course => course.students)
  @JoinTable()
  courses: Course[];
}

Here, the @ManyToMany decorator defines the many-to-many relationship with the Course entity, and course => course.students specifies the corresponding property in the other entity. @JoinTable() specifies the join table for the many-to-many relationship.

1. Course Entity

typescript
import { Entity, PrimaryGeneratedColumn, Column, ManyToMany } from 'typeorm';
import { Student } from './student.entity';

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

  @Column()
  title: string;

  @ManyToMany(() => Student, student => student.courses)
  students: Student[];
}

In the Course entity, we also use @ManyToMany to define the many-to-many relationship with Student, but we do not need @JoinTable here because the join table is already defined in the Student entity.

Database Migration

Once the entities are defined, TypeORM can automatically generate database migration scripts that create the corresponding tables and the join table. You can use TypeORM's CLI tool to generate and run migration scripts:

bash
typeorm migration:generate -n InitialMigration
typeorm migration:run

This will generate and execute the migration based on your entity definitions, creating the required database tables.

Using Relationships

In your service or controller, you can now use these relationships to add data or query related data:

typescript
async addCourseToStudent(studentId: number, courseId: number) {
  const student = await this.studentRepository.findOne(studentId, {
    relations: ['courses'],
  });
  const course = await this.courseRepository.findOne(courseId);
  student.courses.push(course);
  await this.studentRepository.save(student);
}

async getCoursesForStudent(studentId: number) {
  const student = await this.studentRepository.findOne(studentId, {
    relations: ['courses'],
  });
  return student.courses;
}

This is just a basic example illustrating how to use these defined many-to-many relationships in practical applications. In actual development, you may need to handle more complex business logic and data integrity issues.