无法删除具有多对一关系的父实体

Question

I'm developing a web application with Java, Spring and JPA. I have these two entities, which represent a user and its follow notifications. When a user follows another user, the one who gets followed, receives a follow notification.

@Entity
public class User{
    @Id
    private Long userId;

    @OneToMany(mappedBy = "user",
            cascade = CascadeType.ALL,
            orphanRemoval = true)
    private List<Notification> notifications;
}

@Entity
public class FollowNotification{
    @Id
    private Long followNotificationId;

    @ManyToOne(
            fetch = FetchType.LAZY
    )
    @JoinColumn(name = "id")
    private User user;

    @ManyToOne
    private User follower;

}

I want to delete User and delete also all its notifications, but I want to keep the referenced user of the notification, which is the one who started following the main user. The only problem is that when I try to delete the user I get this error:

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails ( database_name . follow_notification , CONSTRAINT FK8ob9ie1tjkkon036uokkg52j2 FOREIGN KEY ( follower_id ) REFERENCES user ( user_id ))

What am I doing wrong? How can I delete a User with all its notification keeping the other side of the relationship?

Answer 1

you have foreign key constraint follower_id which ensures you can not delete user which is referenced by notifications..you need to set referenced user_id to null for all notifications for a given user on many to one side before deleting the user.

i)remove orphanRemoval = true as well as CascadeType.ALL(CascadeType.ALL, which includes REMOVE) indicates that remove operations should be cascaded automatically to entity objects that are referenced by that field (multiple entity objects can be referenced by a collection field)) in User

@OneToMany(mappedBy = "user") private List<Notification> notifications ;

ii)as well as add nullable = true for user in FollowNotification ie

@JoinColumn(name = "id",nullable = true) private User user;

iii)Now code to delete the user would look something like this

public void deleteUser(int id) {
 // code to get entity manager and then      
Query query = entitymanager.createQuery("Select u " + "from User u " + "where u.userId=+id");

      User user=query.getResultList( ).get(0);

      Set notifications= user.getNotification();
      Iterator<Notification> noteIterator = notifications.iterator();
        while (notification.hasNext()){
            Notification notification= noteIterator.next();
            notification.setUser(null); 
            } 
      entitymanager.remove(user);
    }