Question

I have these models :

class Package(models.Model):
    title = CharField(...)

class Item(models.Model)
    package = ForeignKey(Package)
    price = FloatField(...)

class UserItem(models.Model)
    user = ForeignKey(User)
    item = ForeignKey(Item)
    purchased = BooleanField()

I am trying to achieve 2 functionality with the best performance possible :

1. In my templete I would like to calculate each package price sum of all its items. (Aggregate I assume ?)

2. More complicated : I wish that for each user I can sum up the price of all item purchased. so the purchased = True.

Assume I have 10 items in one package which each of them cost 10$ the package sum should be 100$. assume the user purchase 5 items the second sum should be 50$.

I can easily do simple queries with templetetags but I believe it can be done better ? (Hopefully)

Answer 1

To total the price for a specific package a_package you can use this code

Item.objects.filter(package=a_package).aggregate(Sum('price'))

There is a a guide on how to do these kind of queries, and the aggregate documentation with all the different functions described.

This kind of query can also solve your second problem.

UserItem.objects.filter(user=a_user).filter(purchased=True).aggregate(sum('price'))

You can also use annotate() to attach the count to each object, see the first link above.