pojo code for managing many-to-many relationships

indgosky · **Joined:** Mon Jun 06, 2005 9:26 pm **Posts:** 8

Hibernate version: 2.1
Mapping documents: generated by middlegen

Here's a simple association to illustrate my question:

Code:

(user)-1---*-(user_priv)-*---1-(priv)

Assume the join table has some association-specific data in it, so it needs to exist as a navigable class with two 1-many relationships. The pseudocode to manage these relationships might look like this:

Code:

UserPriv addAssociation(User user, Priv priv) {
    UserPriv assoc = new UserPriv(priv,user);
    user.getUserPrivs().add(assoc);
    priv.getUserPrivs().add(assoc);
}
void removeAssociation(UserPriv assoc) {
    assoc.getPriv().getUserPrivs().remove(assoc);
    assoc.getUser().getUserPrivs().remove(assoc);
    session.delete(assoc);
}

That last line is where my question/problem lies. Everything else could be written into the pojos, requiring no session/transaction logic to be embedded in the method. But the act of breaking an association requires object deletion, which (AFAIK) requires a call on the session -- thus making my pojo no longer "po". So how could I do this so that the pojos can remain DB-code-free?

Basilio · **Posted:** Tue Aug 09, 2005 7:56 pm

We have similar model cross over whole application. We did not use many-to-many mapping at all. What we did, we model that relation in Hibernate like

Code:

(user) has collection of (user_priv) (set)  and (user_priv) has just an attribute(priv) (many-to-one).

You need to diside what direction you will map it (depends on your B logic). In this case just put right cascade option on assiociations. For first one we used all-delete-orphans and between (user_priv) and (priv) none

indgosky · **Joined:** Mon Jun 06, 2005 9:26 pm **Posts:** 8

My example is also using two 1-many relationships. The need we have, which I think invalidates your offered approach, is that the table relationship needs to be bi-directional (sometimes we also want to look at all users having a priv, or want to add/remove a user to a specific a priv group). Cascading a deletion of the association from either direction is "wrong" since both ends are permanent fixtures, and only the associations are made/broken on demand.

I hope that clarifies rather than confuses... Thanks in any case.

Basilio · **Posted:** Wed Aug 10, 2005 12:52 am

indgosky wrote:

My example is also using two 1-many relationships. The need we have, which I think invalidates your offered approach, is that the table relationship needs to be bi-directional (sometimes we also want to look at all users having a priv, or want to add/remove a user to a specific a priv group). Cascading a deletion of the association from either direction is "wrong" since both ends are permanent fixtures, and only the associations are made/broken on demand.

I hope that clarifies rather than confuses... Thanks in any case.

What we found with our app that cascade all-delete-orphans works just fine in almost all casses. At least for us. Usage of it does not mean you will use cascade delete. This cascade option perfectly works for purpose to save child collection. In your case assocation. Hibernane smart enough to figure out what items need to be deleted or inserted. And what you need to do just delete or add associations to collection and call session.save(user). Is`t it simple? No additional functionality that will manage your associations everything done via mappings.