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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_next_number(self):
 """ 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) """


        # 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