docs: def get_without_invoice(self):
          """
          Returns transactions that don't have an invoice.
  
          We filter out transactions that have children, because those
          transactions never have invoices - their children are the ones that
          would each have one invoice.
  
          """
          qs = Transaction.objects.filter(
              children__isnull=True, invoice__isnull=True)
          return qs