How to get total value over previous objects of the same class in Django?

Question

I wrote my own billing software in Django. In that I have a model called bills. Every bill has a price value. Now I want to give the bill model a function that adds up the price values of all previous bills and the current one. This function is meant to give me a number of how much I've earned so far.

But every time I try to implement it I run into the problem that I can't use the bill class within the definition of itself.

What is the best approach to add a function to my bill class that gets all the bill objects (including the current one) and adds up the price values?

Answer 1

I'm sensing that I've misunderstood something, but why don't you just do something like this?

class Bill(models.Model):
    due_date = models.DateField()
    price = models.IntegerField()
    paid = models.BooleanField()

    def get_previous_bills(self):
        return Bill.objects.filter(due_date__lte=timezone.now(), paid=True)

    def get_total_price(self):
        return sum(p.price for p in self.get_previous_bills())