Re: circular references and ORM

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 08 May 2008 07:47:17 -0700

Message-ID:

<48231252$0$3240$7836cce5@newsrazor.net>

Lew wrote:

Daniel Pitts wrote:

In particular for JPA and Hibernate using annotations, you have a
@OneToMany(targetEntity=Monkey.class) Set<Monkey> monkeys; in the
Barrel, and a @ManyToOne Barrel barrel; in the Monkey class.

What's important to remember in Relational theory is that ALL
relations include two endpoints, whether you model that explicitly or
not.

I have trouble remembering who "owns" the relation, so that the other
one can say "mappedBy" in its annotation. Monkey owns the Monkey-Barrel
connection, right? Something like

@OneToMany( targetEntity=Monkey.class, mappedBy="barrel" )
Set <Monkey> monkeys;
?

You *can* do that (and often it makes sense). I believe there is also a
way to have a third table that is just for the relation, something like
Monkey_Barrel (id, Monkey_Id, Barrel_id). Which might make more sense
if semantically neither monkey nor barrel should "own" the reference.
Ofcourse, this does have performance implications :-p

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>