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我正在使用JPA實現EclipseLink和MySQL服務器作爲我的應用程序的持久層。我的問題是,一個已經被保存並且已經擁有一個id的對象正在通過與CascadeType.All的多對一關係被第二次插入。當使用CascadeType.ALL時,JPA會保留兩次相同的對象
這裏的實體:
@Entity
@Table(name = "messages")
public class Message extends DbObject {
@ManyToOne(cascade = CascadeType.ALL)
@JoinColumn(name = "user_id")
public User author;
public String message = "";
public Timestamp timestamp;
// Getters and setters removed ....
}
@Entity
@Table(name = "users")
public class User extends DbObject {
private String name;
// Getters and setters removed ....
}
@MappedSuperclass
public abstract class DbObject {
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE)
private long id;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
}
當我運行下面的代碼,然後我在我的「用戶」表中的兩個條目。但我只想將現有的用戶分配給消息對象,而不是創建新的用戶。
User user = getCurrentUser(); // The user object is already persistent
Message msg = new Message();
msg.setUser(user);
EntityManager em = createEntityManager();
em.getTransaction().begin();
em.persist(msg);
em.getTransaction().commit();
em.close();