Django Aggregate- Division with Zero Values - django

Question

I am using Django's aggregate query expression to total some values. The final value is a division expression that may sometimes feature zero as a denominator. I need a way to escape if this is the case, so that it simply returns 0.
I've tried the following, as I've been using something similar my annotate expressions:
from django.db.models import Sum, F, FloatField, Case, When
def for_period(self, start_date, end_date):
return self.model.objects.filter(
date__range=(start_date, end_date)
).aggregate(
sales=Sum(F("value")),
purchase_cogs=Sum(F('purchase_cogs')),
direct_cogs=Sum(F("direct_cogs")),
profit=Sum(F('profit'))
).aggregate(
margin=Case(
When(sales=0, then=0),
default=(Sum(F('profit')) / Sum(F('value')))*100
)
)
However, it obviously doesn't work, because as the error says:
'dict' object has no attribute 'aggregate'
What is the proper way to handle this?

Answer 1

This will obviously not work; because aggregate returns a dictionary, not a QuerySet (see the docs), so you can't chain two aggregate calls together.
I think using annotate will solve your issue. annotate is almost identical to aggregate, except in that it returns a QuerySet with the results saved as attributes rather than return a dictionary. The result is that you can chain annotate calls, or even call annotate then aggregate.
So I believe something like:
return self.model.objects.filter(
date__range=(start_date, end_date)
).annotate( # call `annotate`
sales=Sum(F("value")),
purchase_cogs=Sum(F('purchase_cogs')),
direct_cogs=Sum(F("direct_cogs")),
profit=Sum(F('profit'))
).aggregate( # then `aggregate`
margin=Case(
When(sales=0, then=0),
default=(Sum(F('profit')) / Sum(F('value')))*100
)
)
should work.
Hope this helps.

Answer 2

I've made it work (in Django 2.0) with:
from django.db.models import Case, F, FloatField, Sum, When
aggr_results = models.Result.objects.aggregate(
at_total_units=Sum(F("total_units")),
ag_pct_units_sold=Case(
When(at_total_units=0, then=0),
default=Sum("sold_units") / (1.0 * Sum("total_units")) * 100,
output_field=FloatField(),
),
)

Answer 3

You can't chain together aggregate statements like that. The docs say:
aggregate() is a terminal clause for a QuerySet that, when invoked,
returns a dictionary of name-value pairs.
It returns a python dict, so you'll need to figure out a way to modify your query to do it all at once. You might be able to replace the first call to aggregate with annotate instead, as it returns a queryset:
Unlike aggregate(), annotate() is not a terminal clause. The output of
the annotate() clause is a QuerySet
As for the division by 0 possibility, you could wrap your code in a try catch block, watching for ZeroDivisionError.