如何使用 Typeorm 向 PostgresQL 数据库添加“关系”？

Question

For example

@Entity()
class Post {

  @Column()
  post_hash: string;

  @Column()
  title: string;

  categorys:  Array<Category> = []; 
}

@Entity()
class Category {

  @Column()
  content: string;

  @Column()
  post_hash: number;

}

I want to query all the category content of the corresponding post through Typeorm.

I tried this method and failed.

this.createQueryBuilder('Post').leftJoinAndMapOne(
            'Post.categorys',
             Category,
            'category',
            'category.post_hash = post.post_hash'
      )

This is my error message. QueryFailedError: relation "post" does not exist

Answer 1

Entities

First of all, you must identify each row with a unique ID using @PrimaryColumn() or@PrimaryGeneratedColumn() decorators.

Each Post entity can have multiple categories and each Category entity is related to multiple posts . This is called a many-to-many relation and must be correctly mapped using the @ManyToMany() decorator. See this for more information.

@Entity()
class Post {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  post_hash: string;

  @Column()
  title: string;

  @ManyToMany(() => Category, category => category.posts)
  @JoinTable()
  categories: Category[];
}

@Entity()
class Category {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  content: string;

  @Column()
  post_hash: number;

  @ManyToMany(() => Post, post => post.categories)
  posts: Post[];
}

Query

Obtain all Post (s) with the related categories .

Using find :

const posts: Post[] = await getRepository(Post)
                             .find({ relations: ['categories'] });

Using QueryBuilder :

const posts: Post[] = await getRepository(Post)
                             .createQueryBuilder('post')
                             .leftJoinAndSelect('post.categories', 'category')
                             .getMany();