Spring Data 和 JPA 中的递归关系？

Question

我的Comment实体是自加入的，有一个subComment集合。

@Entity
public class Comment {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private String id;

@OneToMany(mappedBy = "parentComment", cascade = CascadeType.ALL)
private Set<Comment> subComments = new HashSet<>();

@ManyToOne
@JoinColumn(referencedColumnName = "id")
private Comment parentComment;

在我的addComment方法中

public ResponseEntity<Comment> addComment(Comment comment) {
    Comment currComment = commentRepository.save(comment);
    if (currComment.getParentId() != null) {
        Comment parent = commentRepository.findById(currComment.getParentId()).orElse(null);
        if (parent != null) {
            parent.addSubComment(currComment);
            currComment.setParentId(parent.getId());
            currComment.setParentComment(parent);
            commentRepository.save(parent);
        }
    }
    Comment responseComment = commentRepository.save(currComment);
    return ResponseEntity.ok(responseComment);
}

当我尝试建立反向关系（拥有方）时， comment.setParentComment(parent);导致错误

comment.setParentComment(parent); 导致错误： java.lang.IllegalStateException: Cannot call sendError() after the response has been committed

完整评论实体类

@OneToMany(mappedBy = "parentComment", cascade = CascadeType.ALL)
private Set<Comment> subComments = new HashSet<>();

@ManyToOne
@JoinColumn(referencedColumnName = "id")
private Comment parentComment;

private boolean isParent;
private String parentId;

public String getParentId() {
    return parentId;
}

public void setParentId(String parentId) {
    this.parentId = parentId;
}

public Set<Comment> getSubComments() {
    return subComments;
}

public void setSubComments(Set<Comment> subComments) {
    this.subComments = subComments;
}

public Comment addSubComment(Comment comment) {
    this.subComments.add(comment);
    return this;
}

public Comment getParentComment() {
    return parentComment;
}

public void setParentComment(Comment parentComment) {
    this.parentComment = parentComment;
}

public boolean getIsParent() {
    return isParent;
}

public void setIsParent(boolean isParent) {
    this.isParent = isParent;
}

Answer 1

@ManyToOne
@JoinColumn(referencedColumnName = "id")
@JsonBackReference
private Comment parentComment;

我添加了解决java.lang.IllegalStateException: Cannot call sendError() after the response has been committed错误的@JsonBackReference 。父母Comment也能够看到该subComments集合。

Answer 2

你似乎把它复杂化了。

1) 你有@JoinColumn(referencedColumnName = "id")但那是多余的。无论如何，这就是外键引用的内容，因此您无需明确说明。不用担心，但不要编写不需要的代码。

2) 如果您在新的子评论中有 parentId，则不需要查找它并将其添加到父评论的列表中。您在这里缺少的概念是“拥有”实体概念。查看 Javadoc for mappedBy. 由于该parentComment字段进行映射，因此它定义了拥有实体。当然，它是同一个实体，但关键是它parentComment是控制持久性的字段。您无需向Setof添加任何内容即可subComments保持关系。如果需要，您可以这样做，但 JPA 会忽略它。您只需要设置parentComment字段。例如

编辑：这个例子是用 JPA 而不是 Spring Data，但它在引擎盖下是一样的。

您的实体只需：

@Entity
public class Comment {
    @Id @GeneratedValue
    private Integer id;

    @OneToMany(mappedBy="parentComment")
    private Set<Comment> subComments;

    @ManyToOne
    private Comment parentComment;

你像这样使用它：

private void run() {
    runWrite();
    runRead();
    Comment comment = new Comment();
    comment.setId(1);
    Comment subComment = new Comment();
    subComment.setParentComment(comment);
    runSaveSubComment(subComment);
    runRead();
}
private void runWrite() {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("persistence");
    em = emf.createEntityManager();
    tx = em.getTransaction();
    try {
        tx.begin();
        Comment comment = new Comment();
        Comment subComment = new Comment();
        subComment.setParentComment(comment);
        em.persist(comment);
        em.persist(subComment);

        tx.commit();
    } finally {
        emf.close();
    }        
}
private void runRead() {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("persistence");
    em = emf.createEntityManager();
    try {
        Comment comment = em.createQuery("select c from Comment c left join fetch c.subComments where c.id = :id", Comment.class).setParameter("id", 1).getSingleResult();
        System.out.println(comment + Arrays.toString( comment.getSubComments().toArray()) );
    } finally {
        emf.close();
    }
}
private void runSaveSubComment(Comment subComment) {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("persistence");
    em = emf.createEntityManager();
    tx = em.getTransaction();
    try {
        tx.begin();
        em.persist(subComment);
        tx.commit();
    } finally {
        emf.close();
    }        
}

或者如果您想使用 Spring Data。

Comment comment = new Comment();
Comment sub1 = new Comment();
sub1.setParentComment(comment);
repo.save(comment);
repo.save(sub1);
Comment parentComment = repo.fetchSubCommentsById(1);
System.out.println(parentComment + Arrays.toString(parentComment.getSubComments().toArray()));
Comment sub2 = new Comment();
sub2.setParentComment(parentComment);
repo.save(sub2);
parentComment = repo.fetchSubCommentsById(1);
System.out.println(parentComment + Arrays.toString(parentComment.getSubComments().toArray()));
// or 
Comment p = new Comment();
p.setId(1);
Comment sub3 = new Comment();
sub3.setParentComment(p);
repo.save(sub3);
parentComment = repo.fetchSubCommentsById(1);
System.out.println(parentComment + Arrays.toString(parentComment.getSubComments().toArray()));