The power of a Java persistence solution like DataNucleus is demonstrated when persisting relationships between objects. There are many types of 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
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);
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. DataNucleus has two ways of handling this situation. If you have the persistence property "datanucleus.manageRelationships" set to true then it will make sure that the other side of the relation is set correctly, correcting obvious omissions, and giving exceptions for obvious errors. If you set that persistence property to false then it will assume that your objects have their bidirectional relationships consistent and will just persist what it finds.