查询多对多联接表时出现Jpa休眠问题

Question

我在POST和TAG之间有一种多对多的关系。每个帖子可以有很多标签，每个标签可以有很多帖子。

我想管理与映射到联接表的实体的关系。所以我有三个实体类：Post，Tag，PostTag。PostTag具有@ManyToOne注释的Post和Tag成员。

PostTag及其嵌入式密钥：

@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; }
}

当我需要带有给定标签的帖子时，我想使用这个查询：

@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创建了这个选择：

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]

它怎么了？

全部内容都在这里：

https://github.com/riskop/jpa_hibernate_spring_boot_many_to_many_managed_on_join_table_problem

Answer 1

当您使用@MapsId时，hibernate实际上忽略了@Embeddable类相应字段的@Column注释中提供的列名，并开始使用@JoinColumn中的名称。如果没有@JoinColumn，则应用默认约定(引用关系属性的名称的串联；被引用主键列的名称(。

您有：

@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，您可以通过以下方式修复映射：

@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。