Set Many-to-Many relationship decorator for two entities vs create a third entity?

Issue

In TypeORM, suppose I use MySQL database and there are two entities: Student and Course.

A student can learn many course, and a course can be enrolled by many student.

So there are two ways to solve n-n relationship:

• Method #1: the official documents suggests me to add @ManyToMany and @JoinTable decorators into one of them. However, I find it hard to maintain the code when working with a large-scaled backend.

// src/entity/Student.ts
@Entity()
export class Student {
  //...
}

// src/entity/Course.ts
@Entity()
export class Course {
  //...
  @ManyToMany(() => Student)
  @JoinTable()
  students: Student[];
}

• Method #2: Create an entity (like StudentCourse) that link between them. TypeORM requires me to add a primary key into the middle table. This additional step will be time-wasting to add a primary key into all middle tables in a schema and this will increase size capacity.

// src/entity/Student.ts
@Entity()
export class Student {
  //...
  @OneToMany(type => StudentCourse, studentCourse => studentCourse.student)
  studentCourses!: StudentCourse[];
}

// src/entity/Course.ts
@Entity()
export class Course {
  //...
  @OneToMany(type => StudentCourse, studentCourse => studentCourse.course)
  studentCourses!: StudentCourse[];
}

// src/entity/StudentCourse.ts
@Entity()
export class StudentCourse {
  @PrimaryGeneratedColumn({ type: "int", name: "id" })
  id!: number;

  // Some custom columns...

  @ManyToOne(type => Student, student => student.studentCourses)
  @JoinColumn([{ name: "student_id", referencedColumnName: "id" }])
  student: Student;

  @ManyToOne(type => Course, course => course.studentCourses)
  @JoinColumn([{ name: "course_id", referencedColumnName: "id" }])
  course: Course;
}

What I want to know is: Which method should I use to deal with Many-to-Many relationship? When and why? What are the pros and cons between them?

Solution

For the first method @ManyToMany:
We use it when we don't have any other properties we weed between that relationship we want to build,
in your case this method suits you

Ps: @ManyToMany create an association table for us

2/ We use the second method is when we need to add more information for example if need to add a classroom, date then creating a new entity is a must.

Answered By - Youba