DISTINCT ON and update() does the wrong thing

[Daniel Keller]

If I have a model like

from django.db import models
from django.utils import timezone

class Foo(models.Model):
    num = models.IntegerField(default=0)
    date = models.DateTimeField(default=timezone.now)
    flag = models.BooleanField(default=False)

and (with Postgresql) I do

Foo.objects.order_by('num', '-date').distinct('num').only('pk')

I get a query like

SELECT DISTINCT ON ("app_foo"."num") "app_foo"."id" AS Col1 FROM "app_foo" ORDER BY "app_foo"."num" ASC, "app_foo"."date" DESC; args=()

which returns the latest Foo for each num.

BUT, if I do

Foo.objects.order_by('num', '-date').distinct('num').update(flag=True)

then it executes

UPDATE "app_foo" SET "flag" = true; args=(True)

which updates everything.

I don't necessarily think that this behaviour should be supported, but it would be nice to at least get a NotImplementedError.

[Anvesh Mishra]

Should we add a support for this behavior or stick to NotImplementedError ?

[Anvesh Mishra]

On looking into it further it seems like the problem comes because of how the QuerySet.update() works

 query = self.query.chain(sql.UpdateQuery)

The Query.chain() changes the class of the current object to UpdateQuery which removes the previous query as a new object is created i.e

SELECT DISTINCT ON ("distinct_foo"."num") "distinct_foo"."id", "distinct_foo"."num", "distinct_foo"."date", "distinct_foo"."flag" FROM "distinct_foo" ORDER BY "distinct_foo"."num" ASC, "distinct_foo"."date" DESC

and then replaces it with

UPDATE "app_foo" SET "flag" = true; args=(True)

instead of combining both.