Spring + JPA for many relationships

Question

I am very new to Spring, and I try to do many-to-many work as I expect. Relations work fine, tables are created, and data is inserted correctly. What I expect is that when I empty the List (that is, I release the ArrayList users from the Group object), I expect the system to remove the relationships from the database, but this is not the case.

I have the following classes:

@Entity @Table(name = "`group`") public class Group { @Id @GeneratedValue @Column(name = "id") private int id; @Column(name = "name") private String name; @ManyToMany(cascade = {CascadeType.ALL}) @JoinTable( name = "`user_has_group`", joinColumns = @JoinColumn(name = "group_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "user_id", referencedColumnName = "id") ) private List<User> users = new ArrayList<User>(); ... } @Entity @Table(name = "`user`") public class User { @Id @GeneratedValue @Column(name = "id") private int id; @Column(name = "name") private String name; @ManyToMany(mappedBy = "users") private List<Group> groups = new ArrayList<Group>(); ... } 

Here is the DAO:

 @Repository public class GroupJpaDao implements GroupDao { private EntityManager em; @Transactional public void save(Group group) { this.em.merge(group); } ... @PersistenceContext void setEntityManager(EntityManager entityManager) { this.em = entityManager; } } @Repository public class UserJpaDao implements UserDao { private EntityManager em; @Transactional public void save(User user) { this.em.merge(user); } ... @PersistenceContext void setEntityManager(EntityManager entityManager) { this.em = entityManager; } } 

Here is a test method:

 @Test public void test() { Group g = new Group(); g.setName("Test group"); groupDao.save(g); // Works fine - inserts the group into the database User u = new User(); u.setName("Test user"); userDao.save(u); // Works fine - inserts the user into the database g.addUser(u); groupDao.save(g); // Works fine - adds the many-to-many relationship into the database g.removeAllUsers(); groupDao.save(g); // Doesn't work - I'm expecting it to remove all the related relationships from the database but it doesn't! } 

Am I doing something wrong or is it impossible to do?

Any tips are really appreciated.

Thanks!

Answer 1

I am re-reading your question and now the answer is clear. Create a group g and save it. But since your save method uses merge , and you don't take into account the value returned by merge to assign it to g , you continue to merge the transition group g, which never has an identifier. Thus, every time a merge is called, you are actually creating a new group, rather than modifying a previously created one.

Change the save method to

 public Group save(Group group) { return this.em.merge(group); } 

and always reassign the result to g :

 g = groupDao.save(g); 

Of course, the same should be done for the user.

Answer 2

you remove users from the group, which is not enough. you need to remove all users from the group and remove this group from the list of all these groups.