[Django] #26211: prefetch_related with Prefetch with queryset with explicit Ordering Ignored

[Django]

#26211: prefetch_related with Prefetch with queryset with explicit Ordering 
Ignored
-------------------------------+--------------------
     Reporter:  cancan101      |      Owner:  nobody
         Type:  Uncategorized  |     Status:  new
    Component:  Uncategorized  |    Version:  1.9
     Severity:  Normal         |   Keywords:
 Triage Stage:  Unreviewed     |  Has patch:  0
Easy pickings:  0              |      UI/UX:  0
-------------------------------+--------------------
 Given these models:

 {{{
 class Parent(models.Model):
     pass

 class Child(models.Model):
     saved_dt = models.DateTimeField(auto_now_add=True)
     parent = models.ForeignKey(Parent)
 }}}

 I am executing this query (for each `Parent` I want to get the `child_set`
 in sorted order):

 {{{
 [m.child_set.order_by('saved_dt') for m in
 Parent.objects.prefetch_related(Prefetch('child_set',
 Child.objects.order_by('saved_dt'))).all()]
 }}}

 I would expect that to be two SQL queries, but in reality I see `N+2`
 where `N` is the number of `Child`s:


 {{{
 SELECT "prefetch_parent"."id" FROM "prefetch_parent"; args=()
 SELECT "prefetch_child"."id", "prefetch_child"."saved_dt",
 "prefetch_child"."parent_id" FROM "prefetch_child" WHERE
 "prefetch_child"."parent_id" IN (1, 2) ORDER BY
 "prefetch_child"."saved_dt" ASC; args=(1, 2)
 SELECT "prefetch_child"."id", "prefetch_child"."saved_dt",
 "prefetch_child"."parent_id" FROM "prefetch_child" WHERE
 "prefetch_child"."parent_id" = 1 ORDER BY "prefetch_child"."saved_dt" ASC
 LIMIT 21; args=(1,)
 SELECT "prefetch_child"."id", "prefetch_child"."saved_dt",
 "prefetch_child"."parent_id" FROM "prefetch_child" WHERE
 "prefetch_child"."parent_id" = 2 ORDER BY "prefetch_child"."saved_dt" ASC
 LIMIT 21; args=(2,)
 }}}

--
Ticket URL: <https://code.djangoproject.com/ticket/26211>
Django <https://code.djangoproject.com/>
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