How to retrieve the Id of an entity persisted via cascade with JPA

Question

If one has a regular Foo entity and call persist(foo) the @Id is automatically set on the entity. However, if the said Foo entity has a collection of Bars bound to it via

(...)
@OneToMany(cascade=Cascade.ALL) 
Set<Bar> getBars()
(...)

and such instance is already persisted, if one creates a new Bar and add it to the Bars collection of foo and calls merge(foo) the Bar just created is persisted, but its @Id is not updated!

Is there a way to retrieve this id, without making a later call to find(Foo.class, foo.getId())?

Answer 1

A call to merge() will not update the objects that are passed in. It will however return the updated objects.

From the Hibernate docs:

http://www.hibernate.org/hib_docs/v3/reference/en/html/objectstate-saveorupdate.html

and merge() is very different:

• if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance
• if there is no persistent instance currently associated with the session, try to load it from the database, or create a new persistent instance
• the persistent instance is returned
• the given instance does not become associated with the session, it remains detached