[Zope] referential integrity

hi zopistas, Is there a way in zope to define restrictions as we now it from relational databases. In other words, is it possible to store only a reference of an object in a attribute of another object. Additionally, the zope database wouldn't allow to delete the referenced object while it's still referenced (in a attribute of another object). if this is somehow possible, it would make zope very, very powerfull. thanx, lukas