code: def buy_product(self, product_pk):
          
          if self.invoice_sales.filter(line_invoice_sales__line_order__product__pk=product_pk).exists() \
                  or self.ticket_sales.filter(line_ticket_sales__line_order__product__pk=product_pk).exists():
              return True
          else:
              return False
docs: determina si el customer ha comprado un producto