Java Persistent Objects - JPA Relationships

Relationships

The power of a Java persistence solution like JPOX is demonstrated when persisting relationships between objects. There are many types of relationships.

1-1 relationships - this is where you have an object A relates to a second object B. The relation can be unidirectional where A knows about B, but B doesnt know about A. The relation can be bidirectional where A knows about B and B knows about A.
1-N relationships - this is where you have an object A that has a collection of other objects of type B. The relation can be unidirectional where A knows about the objects B but the Bs dont know about A. The relation can be bidirectional where A knows about the objects B and the Bs know about A
N-1 relationships - this is where you have an object B1 that relates to an object A, and an object B2 that relates to A also etc. The relation can be unidirectional where the A doesnt know about the Bs. The relation can be bidirectional where the A has a collection of the Bs. [i.e a 1-N relationship but from the point of view of the element]
M-N relationships - this is where you have objects of type A that have a collection of objects of type B and the objects of type B also have a collection of objects of type A. The relation is always bidirectional by definition

Assigning Relationships

When the relation is unidirectional you simply set the related field to refer to the other object. For example we have classes A and B and the class A has a field of type B. So we set it like this

A a = new A();
B b = new B();
a.setB(b); // "a" knows about "b"

When the relation is bidirectional you have to set both sides of the relation. For example, we have classes A and B and the class A has a collection of elements of type B, and B has a field of type A. So we set it like this

A a = new A();
B b1 = new B();
a.addElement(b1); // "a" knows about "b1"
b1.setA(a); // "b1" knows about "a"

So it is really simple, with only 1 real rule. With a bidirectional relation you must set both sides of the relation

Persisting Relationships - Reachability

To persist an object with JPA you call the EntityManager method persist. The object passed in will be persisted. By default all related objects will not be persisted with that object. You can however change this by specifying the cascade PERSIST property for that field. With this the related object(s) would also be persisted (or updated with any new values if they are already persistent). This process is called persistence-by-reachability. With JDO the default is to persist all reachables, whereas with JPA you have to explicitly set this behaviour. For example we have classes A and B and class A has a field of type B and this field has the cascade property PERSIST set. To persist them we could do

A a = new A();
B b = new B();
a.setB(b); // "a" knows about "b"
em.persist(a);

Managed Relationships

As we have mentioned above, it is for the user to set both sides of a bidirectional relation. If they dont and object A knows about B, but B doesnt know about A then what is the persistence solution to do ? It doesnt know which side of the relation is correct. JPA doesnt define the behaviour currently for this situation. JPOX currently just tries to persist the relation information as presented. If the two sides of a bidirectional relation are inconsistent then the persistence results are undefined.