def tree_post_save_handler(sender, instance, **kwargs):
 instance.get_descendants(include_self=True).update(_ct_inventory=None)

def tree_post_save_handler ( sender , instance , ** kwargs ) : instance . get_descendants ( include_self = True ) . update ( _ct_inventory = None )
clobber the _ct_inventory attribute of this object and all sub-objects on save .