java - 为父实体 JPA-GAE 分配现有子实体

Question

我有 2 个实体，它们之间有关系。

两个实体都已创建并插入到数据库中。 在某些时候，用户能够在它们之间进行连接。 当我尝试这样做时，我得到:

javax.persistence.PersistenceException: Detected attempt to establish WannaMeetUser("654321") as the parent of WannaMeetUser("123456") but the entity identified by WannaMeetUser("123456") has already been persisted without a parent. A parent cannot be established or changed once an object has been persisted.

这是一个传递关系(用户可以有很多来自用户之王的 friend ):

附上代码:

    @Entity 
    public class WannaMeetUser {

        @Id //signifies the primary key
        @Column(name = "ID", nullable = false)
        private Key id;

          @ManyToMany
          @Basic
          private List<WannaMeetUser> userFriends = new ArrayList<WannaMeetUser>();
}

public void addFriendToWannaMeetUser(@Named("userId") String userId,
            @Named("friendId") String friendId) {
        EntityManager mgr = getEntityManager();
        try
        {
            WannaMeetUser user = mgr.find(WannaMeetUser.class, WannaMeetServerUtils.getKeyFromString("WannaMeetUser", userId));
            WannaMeetUser friend = mgr.find(WannaMeetUser.class, WannaMeetServerUtils.getKeyFromString("WannaMeetUser", friendId));
            String coupleId = getcoupleId(userId.toString(), friendId.toString());
            if (friend == null || user == null) {
                throw new EntityNotFoundException("Object does not exist");
            }
            WannaMeetCouple couple=mgr.find(WannaMeetCouple.class, coupleId);
            if (couple == null) {
                couple = createCouple(userId.toString(), friendId.toString());
                couple.setId(coupleId);
                setUserJoined(couple, userId.toString(), friendId.toString(), true);
            }
         else {
            if (isFriendAllready(couple, userId.toString(), friendId.toString()))
                ;
            setUserJoined(couple, userId.toString(), friendId.toString(), false);
            doAddFriend(user, friend, 10, 12321321);
            mgr.persist(couple);
            mgr.persist(friend);
            mgr.persist(user);
        }

        } catch (Exception e) {
            e.printStackTrace();
        }
        finally{
            mgr.close();
        }

    }

我的问题是建立这种关系的最佳方式是什么？

谢谢

Answer 1

消息显示用户 1 已经在没有任何父级的情况下被持久化，但现在我们尝试 将用户 2 保留为用户 1 的父级，但已知用户 1 没有父级。

尝试使用级联一次性保存所有内容。您可以像这样声明级联关系:

@ManyToMany(cascade=CascadeType.ALL)
private List<User> userFriends;

然后使用级联一次性保存所有内容:

User user1 = new User();
User user2 = new User();

user1.getUserFriends().add(user2);

// this persists the whole tree in one go
entityManager.persist(user1);