This is a WorkInProgress description of the NHProjectRepository logic in DS.
NHProjectRepository uses a custom method te determine which objects can be deleted from the database. Unfortunately NHibernate can not do this because NHibernate cascades are evaluated very locally. See for example http://fabiomaulo.blogspot.com/2009/09/nhibernate-tree-re-parenting.html Basically NHibernate can delete object B if object A is deleted but it cannot check if object B is in use elsewhere.
The solution:
S - P = O
SessionObjects - ProjectObjects are Orphans. So everything that is in session but not in project can be deleted. Now our save logic look liks this
1) Determine session objects using NHibernate session's entityentries
2) Determine project objects using EntiteWalker (more about this later)
3) Determin orphan objects by substracting 1 -2 and delete them from the session
4) Flush the session.