한명의 user는 많은 post 를 가질 수 있다
user ➡ one to many relationship with the post
주체 엔티티 user가 하나이므로 one to many!

반대로 post ➡ many to one relationship with the user.
많은 post들이 하나의 user(author)을 갖기 때문이다

user와 Post 처럼, many to one and one to many 인 경우 양방향이라고 보며 many to one and one to many는 보통 항상 함께 존재한다.

in case of many to one and one to many relationship, we will consider it as a bidirectional relationship. So we look at one to many and many to one relationships as a case of a bidirectional relationship between two entities.

• foreign key 는 항상 many to one 쪽에 위치한다. join컬럼은 필요없다
  -> 즉, post 테이블에 author(user) foreign key Id 있음

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실습

양방향 One to one 일때랑 비슷함.

Many to One

import {
  Entity,
  ManyToOne,
} from 'typeorm';
import { User } from 'src/users/user.entity';

@Entity()
export class Post {
  
  ...

  @ManyToOne(() => User, (user) => user.posts) // foreign key 존재
  author: User;


}

One to Many

import { Post } from 'src/posts/post.entity';
import { Entity, OneToMany } from 'typeorm';

@Entity()
export class User {
  ...
  @OneToMany(() => Post, (post) => post.author)
  posts: Post[];

}