docs: def sum_queryset(qs: QuerySet, key: str= 'amount', default=Decimal(0)) -> Decimal:
      """
      Returns aggregate sum of queryset 'amount' field.
      :param qs: QuerySet
      :param key: Field to sum (default: 'amount')
      :param default: Default value if no results
      :return: Sum of 'amount' field values (coalesced 0 if None)
      """
      res = qs.aggregate(b=Sum(key))['b']
      return default if res is None else res