Fixed #21760 -- prefetch_related used an inefficient query for reverse FK.

Regression introduced by commit 9777442. Refs #21410.

AUTHORS

@@ -635,6 +635,7 @@ answer newbie questions, and generally made Django that much better:
     Justine Tunney <jtunney@lobstertech.com>
     Maxime Turcotte <maxocub@riseup.net>
     Amit Upadhyay <http://www.amitu.com/blog/>
+    valtron
     Adam Vandenberg
     Geert Vanderkelen
     Vasil Vangelovski

django/db/models/fields/related.py

@@ -521,7 +521,9 @@ class ReverseSingleRelatedObjectDescriptor(object):
         # composite fields. In the meantime we take this practical approach to
         # solve a regression on 1.6 when the reverse manager in hidden
         # (related_name ends with a '+'). Refs #21410.
-        if self.field.rel.is_hidden():
+        # The check for len(...) == 1 is a special case that allows the query
+        # to be join-less and smaller. Refs #21760.
+        if self.field.rel.is_hidden() or len(self.field.foreign_related_fields) == 1:
             query = {'%s__in' % related_field.name: set(instance_attr(inst)[0] for inst in instances)}
         else:
             query = {'%s__in' % self.field.related_query_name(): instances}

docs/releases/1.6.3.txt

@@ -67,5 +67,9 @@ Other bugfixes and changes
 * Fixed :djadmin:`changepassword` on Windows
   (`#22364 <https://code.djangoproject.com/ticket/22364>`_).
 
+* Fixed regression in ``prefetch_related`` that caused the related objects
+  query to include an unnecessary join
+  (`#21760 <https://code.djangoproject.com/ticket/21760>`_).
+
 Additionally, Django's vendored version of six, :mod:`django.utils.six` has been
 upgraded to the latest release (1.6.1).

tests/prefetch_related/tests.py

@@ -4,8 +4,10 @@ from django.core.exceptions import ObjectDoesNotExist
 from django.contrib.contenttypes.models import ContentType
 from django.db import connection
 from django.db.models import Prefetch
+from django.db.models.query import get_prefetcher
 from django.test import TestCase, override_settings
 from django.utils import six
+from django.utils.encoding import force_text
 
 from .models import (Author, Book, Reader, Qualification, Teacher, Department,
     TaggedItem, Bookmark, AuthorAddress, FavoriteAuthors, AuthorWithAge,
@@ -1055,3 +1057,19 @@ class Ticket21410Tests(TestCase):
 
     def test_bug(self):
         list(Author2.objects.prefetch_related('first_book', 'favorite_books'))
+
+
+class Ticket21760Tests(TestCase):
+
+    def setUp(self):
+        self.rooms = []
+        for _ in range(3):
+            house = House.objects.create()
+            for _ in range(3):
+                self.rooms.append(Room.objects.create(house = house))
+
+    #@override_settings(DEBUG=True)
+    def test_bug(self):
+        prefetcher = get_prefetcher(self.rooms[0], 'house')[0]
+        queryset = prefetcher.get_prefetch_queryset(list(Room.objects.all()))[0]
+        self.assertNotIn(' JOIN ', force_text(queryset.query))