A very useful operation that we recently added in Hibernate is this one:

<T> T getReference(T object);

It's job is to turn a reference to a detached entity into a reference associated with the persistence context.

It's just a shortcut way to write:

em.getReference(entity.getClass(),entity.getId())

I know it seems like a completely trivial little thing, but sometimes conveniences like this can be very helpful.