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"
|With a bidirectional relation you must set both sides of the relation|
To persist an object with JPA you call the EntityManager method persist (or merge if wanting to update a detached object). 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 (and/or MERGE) 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. 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); em.persist(a); // "a" and "b" are provisionally persistent
A further example where you don't have the cascade PERSIST set, but still want to persist both ends of a relation.
A a = new A(); B b = new B(); a.setB(b); em.persist(a); // "a" is provisionally persistent em.persist(b); // "b" is provisionally persistent
As we have mentioned above, it is for the user to set both sides of a bidirectional relation. If they don't and object A knows about B, but B doesnt know about A then what is the persistence solution to do ? It doesn't know which side of the relation is correct. JPA doesn't define the behaviour 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.
|When performing management of relations there are some checks implemented to spot typical errors in user operations e.g add an element to a collection and then remove it (why?!). You can disable these checks using datanucleus.manageRelationshipsChecks, set to false.|