Question

I have 2 Hibernate objects Group and User which are related with a ManyToMany relationship.

I use the following mapping

Group.java

private Set<User> users = new HashSet<User>(0);

...

@ManyToMany(cascade = CascadeType.ALL)  
@JoinTable(name = "user_group",
joinColumns = { @JoinColumn(name = "GROUP_ID") },
inverseJoinColumns = { @JoinColumn(name = "USER_ID") })
public Set<User> getUsers() {
    return this.users;
}
...

User.java

private Set<Group> groups = new HashSet<User>(0);

...


@ManyToMany(mappedBy = "users") 
public Set<Group> getGroups() {
    return this.groups;
}

...

DAO.java

public User addUserToGroup (Integer groupId , Integer  userId){
    Group group = (Group) getBeanById(groupId, Group.class); //Here I get the right group
    User user = (User) getBeanById(userId, User.class);//Here I get the right user
    group.getUsers().add(user);
    user.getGroups().add(group);
    save(group);        
    return user;
}

The save method

public EntityBase save(EntityBase transientInstance) {
    Session session = HibernateUtil.getCurrentSession();
    log.debug("persisting  instance");
    try {
        session.saveOrUpdate(transientInstance);
        log.debug("persist successful");
        return transientInstance;
    } catch (RuntimeException re) {
        log.error("save failed", re);
        throw re;
    }
}

My problem is when I add a user to a groupe, no record is added to the association table user_group.

NB : The begin and commit statements are called in a filter Servlet

Answer 1

The code above does what it should do.
My problem was at the display of the list of the users, Hibernate detect a dirty users collections and delete it. What I ended up doing is to change the ownership of the relationship to Group instead of the User, and all behaved normally.