JPA @OneToMany - Set vs List - 使用 Set 时无法从双向关联中删除一个子实体

Question

Not able to delete child entity from OneToMany association if Set is used. Everything works fine if I use List instead of Set

Post.java

@OneToMany(mappedBy = "post", cascade = CascadeType.ALL, orphanRemoval = true)
private List<Comment> comments;

public void addComment(Comment comment) {
    if (comments == null) {
        comments = new ArrayList<>();
    }
    comment.setPost(this);
    comments.add(comment);
}

public void removeComment(Comment comment) {
    if (comments == null || comments.isEmpty()) {
        return;
    }
    comments.remove(comment);
    comment.setPost(null);
}

Comment.java

@ManyToOne(fetch = FetchType.LAZY)
@EqualsAndHashCode.Include
private Post post;

Code for sample application is available on Github

• master branch is using Set and test case fails
• list branch is using 'List` and test case passes

Not sure whether I am making any mistake. Please suggest.

Answer 1

Your test asserts on the size of Post.comments . It is also @Transactional , meaning postRepository.getOne() returns the exact same instance of Post that you already have in the persistence context. This means dbPost == post , and so your problem has nothing to do with JPA, it is purely a Java problem.

Try putting a breakpoint at the comments.remove(comment) line. What you'll likely see is that it returns false for the Set version. That's because hashCode/equals is not implemented correctly. Try using comments.removeIf(element -> element.getId().equals(comment.getId()) instead to see if the problem goes away.

(As a side note: there are multiple ways equals/hashCode can be implemented for JPA entities, but as a rule of thumb, making it compare a referenced entity is not a good idea, especially when that referenced entity is supposed to be lazily fetched).

EDIT:

The reason why the code works for List , and fails for Set is that:

• a List doesn't use hashCode at all,
• while looking up elements, a HashSet uses the hashCode value before equals to determine potential equality
• the hashCode of an element already in the HashSet is not recalculated automatically when that element is updated.

In the test, when you add Comment s to the Post , the hashCode of each comment is calculated based on the Post.id being null . However, as soon as you call entityManager.flush() , Post.id becomes non-null, and the hashCode of the Comment you're trying to remove changes. Hence, remove cannot find an element in the Set with the same (original) hashCode as the (newly calculated) hashCode for the comment passed as the parameter. This is desipte the fact that equals would have returned true for one of the elements . A List does not have this problem, because all it ever uses for element comparison is equals .