Annotate a sum of two fields multiplied

Question

I have three models, simplified for the example:

class Customer(models.Model):
    email = models.CharField(max_length=128)

class Order(models.Model):
    custo         


        

Answer 1

You could try using a property in the LineItem model:

class Lineitem(models.Model):
    order = models.ForeignKey(Order)
    quantity = models.IntegerField(blank=True)
    price = models.DecimalField(max_digits=6, decimal_places=2)
    def _get_total(self):
        return quantity * price
    total = property(_get_total)

Then I think you can annotate with the total amount spent using

Customer.objects.filter(something).annotate(total_spent=Sum('order__lineitem__total'))

I don't know how the efficiency of this method relates to others, but it is more Pythonic/Django-y than the alternative, which is to write the entire SQL query by hand as in

Customer.objects.raw("SELECT ... from  where ...")