def get_last_tid(self):
        try:
            spec = {'tid': {'$gt': 0}}
            sort = [('tid', -1)]
            row = self.db.docs.find_one(spec, sort=sort)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return row['tid']
        return 0
def _restore_vars(v_list, t_list):
  """Restore variables in v_list from t_list."""
  ops = []
  for v, t in zip(v_list, t_list):
    ops.append(v.assign(t))
  return ops