Calculate and save the total of an invoice in django - django

Question

When I add an invoice, the Total is always 0 but when I update without any changes, it's updated with the totalsubtotals(). I understand that there are many calculations and in my case, the total calculation is done before the subtotals. Any recommendations.
class Invoice(models.Model):
date = models.DateField(default=timezone.now)
client = models.ForeignKey('Client',on_delete=models.PROTECT)
total = models.DecimalField(default=0, max_digits=20, decimal_places=2)
def totalsubtotals(self):
items = self.invoiceitem_set.all()
total = 0
for item in items:
total += item.subtotal
return total
def save(self, *args, **kwargs):
self.total = self.totalsubtotals()
super(Invoice, self).save(*args, **kwargs)
class InvoiceItem(models.Model):
invoice = models.ForeignKey('Invoice', on_delete=models.CASCADE)
product = models.ForeignKey(Product, on_delete=models.PROTECT)
price = models.DecimalField(max_digits=20, decimal_places=2)
quantity = models.DecimalField(max_digits=20, decimal_places=2)
subtotal = models.DecimalField(default=0, max_digits=20, decimal_places=2)
def save(self, *args, **kwargs):
self.subtotal = self.price * self.quantity
super(InvoiceItem, self).save(*args, **kwargs)

Answer 1

It looks to me like your "default = 0" in your InvoiceItem model under subtotal is what's causing the problem, if there is any error with price or quantity the default value is stored, returning 0 to your Invoice model.
I find that default values also make debugging a lot harder so I try to only use them where values are optional, in the case of an invoice, you can't order no quantity of products and you can't have no price either (0 is a number) errors in the input would set the value in the DB to Null (or None in the case of Python) and then your default sets the subtotal to 0.
Removing the default would cause errors when you try to input your values and you can better track down where the issue is based on your error messages.
Alternatively, in your save function for InvoiceItem, you can try...
if self.price && self.quantity: (check that they're not Null/None)
self.subtotal = self.price * self.quantity
else:
raise ValueError('Incorrect values in price or subtotal')