def test_invoice_line_quantity_change(self):

        purchase = self.create_purchase(self.product_fifo,
                                        quantity=13.0,
                                        unit=self.unit_ton,
                                        unit_price=Decimal(1000), invoice_method='shipment')

        self.supplier_shipment_ship(purchase)

        for invoice in purchase.invoices:
            invoice.invoice_date = datetime.date.today()
            invoice.save()

        invoice, = purchase.invoices
        invoice_line, = invoice.lines

        invoice_line.quantity = Decimal('12.')
        invoice_line.save()