Entity with composite foreign keys identifiers should be persisted after related entities without exception

I've seen that when you create an entity with a composite foreign key as identifier it cannot be flushed until the related entities are already flushed to the database and not just persisted.

It would be nice to let the user flush all the entities together and just INSERT first the related entities to get the ID and then use that to INSERT the entity with composite foreign keys.

I'm going to create a pull request with the failing test.