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 _copy_vars(v_list):
  """Copy variables in v_list."""
  t_list = []
  for v in v_list:
    t_list.append(tf.identity(v))
  return t_list