Multiply two fields together in django orm

Question

How would I do the following query (in pseudocode):

CueCompletion.objects.filter(
    user_id=profile_user.pk, status_action_taken_on__gte=day_ago
).sum('amount' * 'opportunities')

Basically, I want to get the sum of the amount * opportunities for the above query.

Answer 1

One possible way to achieve it is to use a Queryset method called values_list .

It returns a particular queryset (a ValuesListQuerySet), and then you can make some further computations on it.

You can do this:

vqs = CueCompletion.objects.filter( user_id=profile_user.pk, status_action_taken_on__gte=day_ago ).values_list('amount','opportunities')

and then you will have something like vqs = [(50, 2), (100, 4), (25, 2), ...] , ie a "list" of n tuples, each with the value of amount and opportunities field.

(actually vqs it's not a list, it's a ValuesListQuerySet, but doesn't matter for our next move).

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Our next move (aka the further computations) is:

total = sum(t[0] * t[1] for t in vqs)

where we are using the built-it sum() function on a generator containing all our ( amount * opportunities ) results.

(why using a generator instead of a list comprehension? check this answer !)

Answer 2

You could also add an annotation to the queryset while using the F object. This way you are doing all the calculations in the database which could be faster than pulling all the values into a list and doing a list comprehension using python memory.

CueCompletion.objects.filter(
user_id=profile_user.pk, status_action_taken_on__gte=day_ago
).annotate(total=F('amount') * F('opportunities'))

Now you can access this value by the total variable.