Django, Model with "Def Self" value, SUM aggregate not working

I have the following model:

class PurchaseOrderLine(models.Model):

 productcode = models.ForeignKey(OurProduct, on_delete=models.PROTECT)

 price = models.DecimalField (max_digits=6, decimal_places=2)

 qty = models.IntegerField()


 def linetotal(self):

    from decimal import *

    total = (self.price * self.qty)

    return total

In my VIEWS.PY I am trying to total the linetotal's:

 tot=PurchaseOrderLine.objects.aggregate(total=Sum('linetotal'))['total'] 

 return HttpResponse(tot)

But it returns FIELDERROR "Cannot resolve keyword 'linetotal' into 开发者_运维技巧field"???

In the query I can replace Sum('linetotal') for Sum('price') and it work fine, but not with the def linetotal(self).

---

The linetotal property doesn't exist at the database level, so how would the ORM handle it? You need to implement the query using extra:

for purchase_order_line in PurchaseOrderLine.objects.extra(select={'total': 'price * qty'}):
    print purchase_order.total