JPA transactions and entity keys

[Peter]

Hi

I'm using JPA in my application. Yesterday I tried to delete some Entities with a same class in one transaction, but I got an exception which said that I can not do this in one transaction as they are in a different entity group. I thought If I persist two entities with the same class they will be in the same entity group, but now I know that they won't. 

The question is how can I persist two entities to be in the same group? 

My  entity looks like this:

@Entity

public class SomeEntity {

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

protected Key key;

}

When I persist them the key is set automatically:

entityManager.persist(someEntityObject);

Deleting them:

... EntityManager em;

... List<SomeEntity> toDelete;

EntityTransaction tx = em.getTransaction();

try {

tx.begin();

for (SomeEntity entity : toDelete) {

em.remove(entity);

} 

tx.commit();

} finally {

if (tx.isActive()) {

tx.rollback();

}

em.close();

}

I saw somehow I can set a parent key to the keys but I think in this way I'll loose the automatic id generation.

Is there a good way to resolve this problem?

Thanks,

Peter

[Michael Hermus]

I don't use JPA myself, so I don't have a code example for you, but you have the right concept at the end of your post. In order to be in the same Entity Group, entities must share the same Parent entity. You should make sure that this is actually sensible in your data model, and not simply do it to support arbitrary transactions. You can also use cross-group transactions, but that only supports 5 different entity groups per transaction.

I think that in JPA it is actually a little messier to use parent entities than with some other persistence mechanisms. If you aren't too far along, and you don't mind using an app engine specific library, I would encourage you to look at Objectify: http://code.google.com/p/objectify-appengine/