JPA合并导致外国实体的重复输入 [英] JPA merge results in duplicate entry of foreign entity

问题描述

问题：

有没有人知道怎么合并而没有 EntityManager 试图重新插入外国实体？

Does anyone know how to merge without having EntityManager trying to re-insert the foreign entity?

情景：

只是设置一个与我的案例非常匹配的场景：我有两个实体

Just to set up a scenario that closely matches my case: I have two entities

@Entity
@Table(name = "login", catalog = "friends", uniqueConstraints =
@UniqueConstraint(columnNames = "username"))
public class Login implements java.io.Serializable{

   private static final long serialVersionUID = 1L;
   @Id
   @GeneratedValue(strategy = IDENTITY)
   @Column(name = "id", unique = true, nullable = false)
   private Integer id;
   @Column(name = "username", unique = true, nullable = false, length = 50)
   private String username;
   @Column(name = "password", nullable = false, length = 250)
   private String password;
}

@Entity
@Table(name = "friendshiptype", catalog = "friends")
public class FriendshipType implements java.io.Serializable{

   private static final long serialVersionUID = 1L;
   @Id
   @GeneratedValue(strategy = IDENTITY)
   @Column(name = "id", unique = true, nullable = false)
   private Integer id;
   @OneToOne(fetch = FetchType.LAZY)
   @JoinColumn(name = "username")
   private Login login;
      @Column(name = "type", unique = true, length = 32)
   private String type;
   ...//other fields go here
}

登录实体和 FriendshipType 实体分别持久保存到数据库。然后，稍后，我需要将 Login 行与 FriendshipType 行合并。当我调用 entityManager.merge（友谊）时，它会尝试插入一个新的登录，这当然会导致以下结果错误

Both the Login entity and the FriendshipType entity are persisted to the database separately. Then, later, I need to merge a Login row with a FriendshipType row. When I call entityManager.merge(friendship), it tries to insert a new Login which of course results in the following error

Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry 'myUserName1350319637687' for key 'username'
Error Code: 1062
Call: INSERT INTO friends.login (password, username) VALUES (?, ?)

我的问题是，如何在没有enityManager尝试重新插入外来对象的情况下合并两个对象？

My question, again, is how do I merge two objects without having enityManager trying to reinsert the foreign object?

推荐答案

以下是我解决问题的方法。我终于想出合并没有解决的原因是因为login.id是由JPA自动生成的。因为我真的不需要自动生成的id字段，我将其从架构中删除并使用 username 作为 @id 字段：

Here is how I solve the problem. I finally figure the reason the merge is not resolving is because the login.id is auto generated by JPA. So since I really don't need an auto-generated id field, I remove it from the schema and use username as the @id field:

@Entity
@Table(name = "login", catalog = "friends", uniqueConstraints =
@UniqueConstraint(columnNames = "username"))
public class Login implements java.io.Serializable{

   private static final long serialVersionUID = 1L;
   @Id
   @Column(name = "username", unique = true, nullable = false, length = 50)
   private String username;
   @Column(name = "password", nullable = false, length = 250)
   private String password;
}

我遇到的另一种解决方案，我没有实施，但可能有所帮助别人，他们是否需要拥有自动生成的id字段。

Another solution that occurred to me, which I didn't implement but may help someone else, should they need to have an auto-generated id field.

而不是创建 Login的实例对于合并，从数据库中获取实例。我的意思是，而不是

Instead of creating an instance of Login for the merger, get the instance from the database. What I mean is, instead of

Login login = new Login(); login.setUsername(username); login.setPassword(password);

宁愿

Login login = loginDao.getByUsername(username);

这样，就不会生成新的id字段，使实体看起来与众不同。

That way, a new id field is not generated making the entity seem different.

感谢所有人的帮助，特别是@mijer非常耐心。

Thanks and up-votes to everyone for helping, especially to @mijer for being so patient.

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