Hibernate在ManyToMany关系中生成其他查询，而没有设置CascadeType.REMOVE

Question

I have the entity User :

@Entity
@Table(name = "users")
public class User {
    (...)
    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "users_roles",
            joinColumns = @JoinColumn(name = "user_id", nullable = false),
            inverseJoinColumns = @JoinColumn(name = "role_id", nullable = false)
    )
    private List<Role> roles;
}

and entity Role with simple id and name columns.

User and Role have many-to-many relation with join table users_roles .

I have created method to remove a user:

public void remove(long userId) {
    Session session = getSession();

    //NativeQuery joinTableQuery = session.createNativeQuery("DELETE FROM users_roles ur WHERE ur.user_id = :userId");
    //joinTableQuery.setParameter("userId", userId);
    //joinTableQuery.executeUpdate();

    Query userQuery = session.createQuery("DELETE FROM User u WHERE u.id = :userId");
    userQuery.setParameter("userId", userId);
    userQuery.executeUpdate();
}

I have commented out first NativeQuery on purpose to check what happens. And what is interesting now Hibernate generates two queries:

• Hibernate: delete from users_roles where (user_id) in (select id from users where id=?)
• Hibernate: delete from users where id=?

Question:

Why does Hibernate generate additional query on users_roles (join table) while my User entity has no CascadeType.REMOVE set on @ManyToMany relation? I thought I have to write it myself (commented part).

Answer 1

Your User entity owns the many-to-many association to the Role entity. When you remove a User , Hibernate automatically also removes all entries from the association table. But it doesn't cascade the remove operation to the Role entity.

You should never use CascadeType.REMOVE on a many-to-many association. If you remove an entity, Hibernate will remove all associated entities even if they are still referenced by other entities. I explained that in great detail on my blog .

If you would use CascadeType.REMOVE on your roles association and remove a User , Hibernate would remove all Role entities referenced by that User . It would do that even if there are other User entity objects that are associated with these Role s.