docs: def is_validated(self):
          """
          Returns True if this instance is validated.
  
          Note that resolving this property requires a DB query, so if you've a
          very large amount of receipts you should prefetch (see django's
          ``select_related``) the ``validation`` field. Even so, a DB query *may*
          be triggered.
  
          If you need a large list of validated receipts, you should actually
          filter them via a QuerySet::
  
              Receipt.objects.filter(validation__result==RESULT_APPROVED)
  
          :rtype: bool
          """
          # Avoid the DB lookup if possible:
          if not self.receipt_number:
              return False
  
          try:
              return self.validation.result == ReceiptValidation.RESULT_APPROVED
          except ReceiptValidation.DoesNotExist:
              return False