how to use annotate to sum up all rows and get the avg? django

Question

I was googling about this and there is a simplier way which uses Cast but then thing is I believe that's for django 2.0 that mine is django 1.9

I found a post and I tried it, it doesn't really work for me though.

Let's say I have a model with value and max fields. I want to get the sum of all value and max then divide them to get the average. What I did just gets one row and return the avg of the row, can someone let me know what I have been doing wrong here?

model_calcuation = Model.objects.filter().annotate(
    sum_score=Sum('value', output_field=FloatField()),
    sum_max=Sum('max', output_field=FloatField())
).annotate(
    avg=F('sum_score') / F('sum_max')
)

Thanks in advance for any help

Answer 1

If I understood correctly your question, you are looking for aggregate() operation.

from django.db.models import F, Sum

model_calcuation = Model.objects.aggregate(result=Sum(F('value')) / Sum(F('max')))