[Django] #28616: DISTINCT ON and update() does the wrong thing

[Django]

#28616: DISTINCT ON and update() does the wrong thing
-------------------------------------+-------------------------------------
               Reporter:  Daniel     |          Owner:  nobody
  Keller                             |
                   Type:  Bug        |         Status:  new
              Component:  Database   |        Version:  1.11
  layer (models, ORM)                |
               Severity:  Normal     |       Keywords:
           Triage Stage:             |      Has patch:  0
  Unreviewed                         |
    Needs documentation:  0          |    Needs tests:  0
Patch needs improvement:  0          |  Easy pickings:  0
                  UI/UX:  0          |
-------------------------------------+-------------------------------------
 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`.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/28616>
Django <https://code.djangoproject.com/>
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