Who's responsability is it to check constraints? In the past I make the RDBMS do all the work of checking things like relationships, and then let the application know that something isn't allowed.
From the docs:
So a typo could mean closing the session? I'm not saying that's wrong, but it indicates to me that in the Hibernate world, either closing a session is thought of as no biggie, or it is considered a bug to try and update the DB with invalid data.

My question likely comes from a lack of understanding when a Sesssion should start and end...so I'll be back in the 'forum mines' if anybody needs me.