many-to-many as two one-to-many 's

baconrad · **Joined:** Tue Sep 21, 2004 6:35 pm **Posts:** 2

The Hibernate In Action book recommends that many-to-many relationships are often best mapped as two one-to-many 's in several places. I wish they gave an example..

My question regards how to navigate to the desired set. If I have a many-to-many of User to Roles, how do I get a set of Roles when mapped as two one-to-many 's. When I map a one-to-many from User to UserRole, I get a set of UserRoles, not Users.

Thanks for your help (I tried searching for the answer but didn't find it).

nordmann · **Posted:** Tue Sep 21, 2004 9:33 pm

In this situation, once I had a UserRole object, I would usually just call userRole.getUser() or userRole.getRole() to access one entity or the other, depending on which way you are trying to go.

For example, use something like

Code:

Role role = RoleDAO.getById(new Integer(1));
Iterator it = role.getUserRoles();
while (it.hasNext) {
    User user = (User)it.next();
    user.sendEmail("Hi There, " + user.getFirstName());
}

But I'm no expert at all. This may be kludgier than is necessary. Try it and see if it suits your need.

If you want to explore further for something a bit more elegant, you can try putting a many-to-many relationship inside a set or list. But I think this only works in one direction. Hibernate In Action shows (briefly) how to do this.