Modulo in django queryset?

Question

Is there already a way to select objects according to id % 4 == 0 in django queryset ?

The only solution I found so far is to write a custom Transform since modulo seems to be supported in Postgresql but I got errors.

Here is my code:

from django.db.models import Lookup, Transform
from django.db.models.fields import Field, IntegerField, AutoField

class Modulo4(Transform):
    lookup_name = 'modulo4'

    def as_sql(self, compiler, connection):
        lhs, params = compiler.compile(self.lhs)
        return "%s %% 4" % lhs, params

AutoField.register_lookup(Modulo4)

foos = Foo.filter(id__modulo4=0)

And I get an error when the queryset is evaluated (with foos.count() or whatever):

IndexError: tuple index out of range

refering to a django file. It seems to be due to the modulo since I manage to make the custom transform work with the doc example.

Any idea on how to achieve this ?

Answer 1

I don't have Django 1.8 at hand to test it, but looks like you can do this using F with annotation, like this:

Foo.objects.annotate(idmod4=F('id') % 4).filter(idmod4=0)

(Django 1.7 and earlier doesn't support using F in annotate)

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For Django 1.7 and earlier you can always use .extra() but it isn't very pretty:

Foo.objects.extra(where=["id %% %s = 0"], params=[4])