多对多条件查询

Question

我正在尝试实现一个帖子的标签功能。

我正在使用下面显示的表格对问题进行建模：

​

我想进行一个查询，以检索具有某些特定标签的所有帖子。谁能告诉我如何才能做到这一点？

Answer 1

正确地设置数据模型应该能起到作用

@Entity(name = "Post")
@Table(name = "post")
public class PostJpa {

   @Id
   private Long id;

   @ManyToMany
   @JoinTable(name = "post_tag", joinColumns = @JoinColumn(name = "postId"), inverseJoinColumns = @JoinColumn(name = "tag_id"))
   Set<TagJpa> tags;
}

@Entity(name = "Tag")
@Table(name = "tag")
public class TagJpa {

    @Id
    @Nullable
    private Long id;

    @Nullable
    private String name;

    @Nullable
    @ManyToMany(mappedBy = "tags")
    Set<PostJpa> posts;
}

这样，您就可以创建以下简单的连接

public void getPosts() {
    CriteriaBuilder builder = entityManager.getCriteriaBuilder();

    CriteriaQuery<PostJpa> query = builder.createQuery(PostJpa.class);
    Root<PostJpa> root = query.from(PostJpa.class);
    Join<Object, Object> subRoot = root.join("tags", JoinType.INNER);

    query.where(builder.equal(subRoot.get("name"), builder.literal("jpa")));

    entityManager.createQuery(query).getResultList();
}