code: def _get_next_number(self):
          
  
          # Recupere les facture de l annee
          relative_invoices = Invoice.objects.filter(invoice_date__year=self.invoice_date.year)
          # on prend le numero le plus eleve du champs number, sinon on met 0
          last_number = relative_invoices.aggregate(Max('number'))['number__max'] or 0
  
          return last_number + 1
docs: Returnes next invoice number - reset yearly.
  
          .. warning::
  
              This is only used to prepopulate ``number`` field on saving new invoice.
              To get invoice number always use ``number`` field.
  
          .. note::
  
              To get invoice full number use ``invoice_id`` field.
  
          :return: string (generated next number)