Add sums by budgetary item id

    @property

    def by_budgetary_item_id(self):

        return group_by(self, lambda payment_order: payment_order.budgetary_item_id)

    @property

    def sums_by_budgetary_item_id(self):

        return {

            budgetary_item_id: sum(po.amount for po in payment_orders)

            for budgetary_item_id, payment_orders in self.by_budgetary_item_id

        }