CascadeType.DETACH 没有通过 Hibernate 中的 @ManyToMany 关系传播

Question

我正在尝试克隆一个帖子对象。为此，我将其 ID 设置为 null，然后使用 entityManager 将其分离，然后保存。

关于帖子 cmets，我让 CascadeType.DETACH 完成传播更改的工作。考虑下一段代码来说明问题：

// PostService.java
private void clonePost(Post post) {
  post.setId(null);
  post.getComments().forEach(comment -> comment.setId(null));

  entityManager.detach(post); // CascadeType will take care of detaching the comments
  postRepository.save(post);  // now we have a duplicated post with the same comments

  // Since it's a detached entity and has no ID, Hibernate will treat it as a 
  // new entity and save it creating a new record
}

现在，有一个问题。这是Post 类：

public class Post {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @SequenceGenerator(allocationSize = 50, initialValue = 1)
    private Long id;
    
    // ...
    
    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Comment> comments = new ArrayList<>();
    
    // ...
    
    @ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.DETACH)
    @JoinTable(name = "post_tags", 
          joinColumns = @JoinColumn(name = "post_fk", referencedColumnName = "id"), 
          inverseJoinColumns = @JoinColumn(name = "post_tag_fk", referencedColumnName = "id"))
    @Fetch(FetchMode.SUBSELECT)
    private Set<PostTag> tags = new HashSet<>();
}

帖子tags 使用@ManyToMany 的单向关系，Post 是其所有者，并且也是懒惰的获取。通常，分离帖子也会导致标签分离，因为cascade = CascadeType.DETACH设置在@ManyToMany中。

但是，这并没有发生。标签没有被分离。懒惰似乎确实有某种影响，可以防止分离充分传播，因为手动执行或初始化标签可以解决问题：

private void clonePost(Post post) {
    // ...
    
    // option 1 - manual detaching
    post.getTags().forEach(tag -> entityManager.detach(tag)); 
    
    // option 2 - initializing tags allows the detach of the post to propagate to them
    post.getTags().forEach(tag -> Hibernate.initialize(tag));

    entityManager.detach(post);
    postRepository.save(post);
}

如果标签设置为急切加载，这两个选项都可以避免：

public class Post {
    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.DETACH)
    @JoinTable(name = "post_tags", 
          joinColumns = @JoinColumn(name = "post_fk", referencedColumnName = "id"), 
          inverseJoinColumns = @JoinColumn(name = "post_tag_fk", referencedColumnName = "id"))
    // @Fetch(FetchMode.JOIN) // this fetch mode will also make the tags to load eager
    private Set<PostTag> tags = new HashSet<>();
}

为什么会这样？为什么父级分离时CascadeType.DETACH 不传播到延迟加载的@ManyToMany 集合？

Answer 1

我猜它的发生原因与REMOVE 相同。休眠文档says：

对于@ManyToMany 关联，REMOVE 实体状态转换没有意义级联，因为它会传播到链接表之外。由于另一端可能被父端的其他实体引用，因此自动删除可能会以ConstraintViolationException 结束。

例如，如果定义了 @ManyToMany(cascade = CascadeType.ALL) 并且第一个人将被删除，Hibernate 将抛出异常，因为另一个人仍与被删除的地址相关联。

Person person1 = entityManager.find(Person.class, personId);
entityManager.remove(person1);

Caused by: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement
Caused by: java.sql.SQLIntegrityConstraintViolationException: integrity constraint violation: foreign key no action; FKM7J0BNABH2YR0PE99IL1D066U table: PERSON_ADDRESS