Calculate with agregates and save on table field on django - django

Question

How to calculate the total of invoice using sum annotate or agregate instead of forloop and save the value on field invoice.total
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

Please don't. This is a form of data duplication: you store some part of the data twice: both the subtotals and the total. In fact subtotal is not necessary either.
It turns out that keeping data in sync is a hard problem. Imagine that you later change the quantity of an InvoiceItem, then you will not save the Invoice again, and thus the total is no longer correct. If you remove an InvoiceItem or you create a new InvoiceItem, then that is not the case either. You can make use of signals, but a lot of Django ORM calls will not run the signals. Indeed for example:
from django.db.models import F
InvoiceItem.objects.filter(pk=14).update(quantity=F('quantity')+1)
Here we thus increment a quantity, but the subtotal and total will not be updated, not even if we use signals.
You can try to implement this at the database level through triggers, but that will require manually adding triggers, and it is furthermore still likely you forget a scenario then eventually the data will get out of sync.
It is therefore more convenient not to use duplicated data. You can for example calculate the subtotal through a property:
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)
#property
def subtotal(self):
return self.price * self.quantity
for the Invoice you should not calculate this at the Django/Pyton layer, since that will take too much time and result in extra querying. You can use .annotate(…) in the views where you need the total of the Invoices with:
from django.db.models import F, Sum
Invoice.objects.annotate(
total=Sum(F('invoiceitem__quantity') * F('price'))
)
Your Invoice model thus does not contain a total or any subroutines to recalculate the total:
class Invoice(models.Model):
date = models.DateField(default=timezone.now)
client = models.ForeignKey('Client',on_delete=models.PROTECT)