查询多对多连接时 Jpa 休眠问题 table

Question

我在 POST 和 TAG 之间建立了多对多关系。每个post可以有多个标签，每个标签可以有多个post。

我想管理与映射到连接 table 的实体的关系。所以我有 3 个实体类：Post、Tag、PostTag。 PostTag 有 @ManyToOne 注释 Post 和 Tag 成员。

PostTag 及其嵌入密钥：

@Entity
@Table(name = "post_tag")
public class PostTag {
        
  @EmbeddedId
  private PostTagId id;
    
  @ManyToOne(fetch = FetchType.LAZY)
  @MapsId("postId")
  private Post post;
        
  @ManyToOne(fetch = FetchType.LAZY)
  @MapsId("tagId")
  private Tag tag;
    
  private PostTag() {}
        
  public PostTag(Post post, Tag tag) {
    this.post = post;
    this.tag = tag;
    this.id = new PostTagId(post.getId(), tag.getId());
  }
    
  public PostTagId getId() { return id; }
  public void setId(PostTagId id) { this.id = id; }
  public Post getPost() { return post; }
  public void setPost(Post post) { this.post = post; }
  public Tag getTag() { return tag; }
  public void setTag(Tag tag) { this.tag = tag; }
}

@Embeddable
public class PostTagId implements Serializable {
     
  @Column(name = "post_id")
  private Long postId;
     
  @Column(name = "tag_id")
  private Long tagId;
     
  public PostTagId() {}
     
  public PostTagId(Long postId, Long tagId) {
    this.postId = postId;
    this.tagId = tagId;
  }
     
  public Long getPostId() { return postId; }
  public void setPostId(Long postId) { this.postId = postId; }
  public Long getTagId() { return tagId; }
  public void setTagId(Long tagId) { this.tagId = tagId; }
}

当我需要具有给定标签的帖子时，我想使用此查询：

@Query(value = "select pt from PostTag pt join fetch pt.post where pt.id.tagId = :tagId")
Set<PostTag> findByTagIdAndFetchPosts(@Param("tagId") Long tagId);

问题是 Hibernate 创建了这个 select:

    2020-08-25 10:21:57.486 DEBUG 16791 --- [           main] org.hibernate.SQL                        : 
        select
            posttag0_.post_id as post_id1_1_0_,
            posttag0_.tag_tag_id as tag_tag_2_1_0_ 
        from
            post_tag posttag0_ 
        where
            posttag0_.post_id=? 
            and posttag0_.tag_tag_id=?

导致以下错误：

    2020-08-25 10:21:57.487  WARN 16791 --- [           main] o.h.engine.jdbc.spi.SqlExceptionHelper   : SQL Error: 42122, SQLState: 42S22
    2020-08-25 10:21:57.487 ERROR 16791 --- [           main] o.h.engine.jdbc.spi.SqlExceptionHelper   : Column "POSTTAG0_.TAG_TAG_ID" not found; SQL statement:
    select posttag0_.post_id as post_id1_1_0_, posttag0_.tag_tag_id as tag_tag_2_1_0_ from post_tag posttag0_ where posttag0_.post_id=? and posttag0_.tag_tag_id=? [42122-200]

怎么了？

全部内容在这里：

https://github.com/riskop/jpa_hibernate_spring_boot_many_to_many_managed_on_join_table_problem

Answer 1

当你使用@MapsId时，hibernate实际上忽略了@Embeddableclass的适当字段的@Column注释中提供的列名，并开始使用该名称来自 @JoinColumn。如果 @JoinColumn 不存在，则应用默认约定（引用关系名称的串联属性；“_”；引用的主键列的名称）。

你有：

@Entity
public class Tag {

    @Id
    @Column(name="TAG_ID")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    // ...
}

@Entity
@Table(name = "post_tag")
public class PostTag {
    
    @EmbeddedId
    private PostTagId id;

    // There is no @JoinColumn annotation !!!
    // So, the default naming convention is used 
    // "tag" + "_" + "tag_id"
    // "tag_id" is the Tag's entity PK column name
    @ManyToOne(fetch = FetchType.LAZY)
    @MapsId("tagId")
    private Tag tag;
    
    // ...
}

因此，您可以通过这种方式修复映射：

@Entity
@Table(name = "post_tag")
public class PostTag {
    
    @EmbeddedId
    private PostTagId id;

    // by chance the default naming convention 
    // lead to the same column name for this case
    @MapsId("postId")
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "post_id") 
    private Post post;
    
    @MapsId("tagId")
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "tag_id")
    private Tag tag;

    // ...
}

并且您可以从 PostTagId 字段中删除 @Column 注释：

@Embeddable
public class PostTagId implements Serializable {
 
    private Long postId;
 
    private Long tagId;
    // ...
}

因为它们被忽略了。

查询多对多连接时 Jpa 休眠问题 table

Jpa hibernate problem while querying many to many join table

orm

many-to-many

hibernate

jpa