当A和B存在一对多关系时，如何知道TypeORM中最多有多少个B与特定A相关？

Question

在下面的示例中，

BlogPost

实体依赖于

BlogPostCategory

：

import { Entity, Column, PrimaryGeneratedColumn, ManyToOne as ManyToOne, Relation } from "typeorm";
import { isUndefined } from "@yamato-daiwa/es-extensions";


@Entity()
export default class BlogPost {

  @PrimaryGeneratedColumn("uuid")
  public readonly ID!: BlogPost.ID;

  @Column({ type: "varchar", nullable: false })
  public readonly heading!: string;

  @Column({ type: "text", nullable: false })
  public readonly HTML!: string;

  /* [ Theory ] About `Relation` https://stackoverflow.com/a/71983552/4818123 */
  @ManyToOne(
    (): typeof BlogPostCategory => BlogPostCategory,
    (blogPostCategory: BlogPostCategory): ReadonlyArray<BlogPost> => blogPostCategory.blogPosts
  )
  public readonly category!: Relation<BlogPostCategory>;

  @Column({ type: "varchar", nullable: false })
  public readonly metaDescription!: string;

  @Column({ type: "timestamp with time zone", nullable: false })
  public readonly publishingDateTime__ISO8601!: string;

  public constructor(
    properties?: Readonly<Omit<BlogPost, "ID">>
  ) {

    if (isUndefined(properties)) {
      return;
    }


    this.heading = properties.heading;
    this.HTML = properties.HTML;
    this.category = properties.category;
    this.metaDescription = properties.metaDescription;
    this.publishingDateTime__ISO8601 = properties.publishingDateTime__ISO8601;

  }

}

@Entity()
@Unique([ "text" ])
export default class BlogPostCategory {

  @PrimaryGeneratedColumn("uuid")
  public readonly ID!: BlogPostCategory.ID;

  @Column({ type: "varchar", nullable: false })
  public text!: string;

  @OneToMany(
    (): typeof BlogPost => BlogPost,
    (blogPost: BlogPost): BlogPostCategory => blogPost.category
  )
  public blogPosts!: Array<BlogPost>;


  public constructor(properties?: Readonly<Omit<BlogPostCategory, "ID">>) {

    if (isUndefined(properties)) {
      return;
    }


    this.text = properties.text;

  }

}

如何知道有多少帖子属于特定类别？

在

await this.blogPostCategoryRepository.find({ relations: [ "blogPosts" ] });

所有博客文章都将被检索到每个类别，这对性能影响巨大。 目标是知道每个类别有多少博客文章，但所有博客文章的检索都是主动提供的。

Answer 1

你应该在 findOption 中添加 loadRelationIds: true ，它将返回 id 数组

    await this.blogPostCategoryRepository.find({
      loadRelationIds: true,
      relations: ['blogPosts'],
    });