Say I persist an object with auto generated id. How do I get back the new Id?

I have


Then I do


Now how do I figure out the primary key of the "a" I just saved? I need it to relate this object to another by its key. .e.g I have class B with a collection of A's. Without filling in a's id, I can't say em.persist(instanceOfB)

assume that A can only be identified uniquely by primary key.

Just add the A's to the collection in B and then persist B. Hibernate will figure everything out automatically.

Thanks for the quick reply. That would work just for this case. But in general (that is, without assuming everything is done in one transaction), say we have another class C that has a collection of A's as well. (I actually do). The difference is that B owns A's but C doesn't. e.g.


I'm doing the following:
create B, add A to B, persist
some time later
an instance of C needs to have that A I just created.

The problem is the same as before: I can't obtain a fresh version of A because both A and B are only identified uniquely by their own primary keys.

I am not sure that I completely understand your use case... but you can force queued operations to be synchronized with the database by calling EntityManager.flush(). The id value should be available after that.