docs: def check_deleted_nodes(self):
          """ delete imported nodes that are not present in the old database """
          imported_nodes = Node.objects.filter(data__contains=['imported'])
          deleted_nodes = []
  
          for node in imported_nodes:
              if OldNode.objects.filter(pk=node.pk).count() == 0:
                  user = node.user
                  deleted_nodes.append(node)
                  node.delete()
                  # delete user if there are no other nodes assigned to her
                  if user.node_set.count() == 0:
                      user.delete()
  
          if len(deleted_nodes) > 0:
              self.message('deleted %d imported nodes from local DB' % len(deleted_nodes))
          self.deleted_nodes = deleted_nodes