Django's annotate Count with division returns integer instead of float

Question

I have many objects and 3 of them have name='AAA'

I group them by 'name' and annotate num in group:

my_models = MyModel.objects.order_by('name').values('name').annotate(count=Count('name'))

for i in my_models:
    print(i.count, i.name)

I get:

3, 'AAA'
1, 'BBB'
...

Everything is fine, but when I try to add some formula to annotate Count():

my_models = MyModel.objects.order_by('name').values('name').annotate(count=Count('name') / 2)

I get:

1, 'AAA'
0, 'BBB'
...

But expected:

1.5, 'AAA'
0.5, 'BBB'
...

EDIT:

Python division differs from SQL division through Django's ORM, so 2/1 in python 3 returns 2.0 - OK, but not in SQL

@jonrsharpe No, python division and division from SQL differs :( — MaxCore, Dec 11 '16 at 22:16
In Django 1.10 you can use `Cast`, see [this answer](http://stackoverflow.com/a/40550677/113962). On Django 1.8+, you might be able to use [`ExpressionWrapper`](https://docs.djangoproject.com/en/1.8/ref/models/expressions/#using-f-with-annotations). — Alasdair, Dec 11 '16 at 22:39

score 23 · Accepted Answer · edited May 23 '17 at 11:46

23

Full answer following @Alasdair's comment:

from django.db.models import FloatField
from django.db.models.functions import Cast

qs = MyModel.objects.order_by('name').values('name').annotate(
    count=Cast(Count('name') / 2.0, FloatField()))

edited May 23 '17 at 11:46

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answered Dec 18 '16 at 12:20

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@rluts , try to cast both of variable first, example total=Cast(Count('id'), FloatField() / Cast(Count('id'), FloatField()) – taipei Jun 25 '19 at 06:42
It isn't actually necessary to cast both - the way Python works is that if one of the values of an arithmetic expression is a float the result will be cast to float as well. – eega Feb 25 '22 at 13:26

Django's annotate Count with division returns integer instead of float

1 Answers1