[1.6.x] Fixed #21410 -- prefetch_related() for ForeignKeys with related_name='+'

Regression introduced by commit 9777442.

Thanks to trac username troygrosfield for the report and test case.

Backpatch of cb83448891 from master.

Conflicts:

	tests/prefetch_related/models.py

django/db/models/fields/related.py

@@ -273,7 +273,17 @@ class ReverseSingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjec
         rel_obj_attr = self.field.get_foreign_related_value
         instance_attr = self.field.get_local_related_value
         instances_dict = dict((instance_attr(inst), inst) for inst in instances)
-        query = {'%s__in' % self.field.related_query_name(): instances}
+        related_field = self.field.foreign_related_fields[0]
+
+        # FIXME: This will need to be revisited when we introduce support for
+        # 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():
+            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}
+
         qs = self.get_queryset(instance=instances[0]).filter(**query)
         # Since we're going to assign directly in the cache,
         # we must manage the reverse relation cache manually.

tests/prefetch_related/models.py

@@ -215,3 +215,18 @@ class WordEntry(models.Model):
 
     def __str__(self):
         return "%s (%s)" % (self.name, self.id)
+
+
+## Ticket #21410: Regression when related_name="+"
+
+@python_2_unicode_compatible
+class Author2(models.Model):
+    name = models.CharField(max_length=50, unique=True)
+    first_book = models.ForeignKey('Book', related_name='first_time_authors+')
+    favorite_books = models.ManyToManyField('Book', related_name='+')
+
+    def __str__(self):
+        return self.name
+
+    class Meta:
+        ordering = ['id']

tests/prefetch_related/tests.py

@@ -9,7 +9,7 @@ from django.utils import six
 from .models import (Author, Book, Reader, Qualification, Teacher, Department,
     TaggedItem, Bookmark, AuthorAddress, FavoriteAuthors, AuthorWithAge,
     BookWithYear, BookReview, Person, House, Room, Employee, Comment,
-    LessonEntry, WordEntry)
+    LessonEntry, WordEntry, Author2)
 
 
 class PrefetchRelatedTests(TestCase):
@@ -651,3 +651,29 @@ class Ticket19607Tests(TestCase):
 
     def test_bug(self):
         list(WordEntry.objects.prefetch_related('lesson_entry', 'lesson_entry__wordentry_set'))
+
+
+class Ticket21410Tests(TestCase):
+
+    def setUp(self):
+        self.book1 = Book.objects.create(title="Poems")
+        self.book2 = Book.objects.create(title="Jane Eyre")
+        self.book3 = Book.objects.create(title="Wuthering Heights")
+        self.book4 = Book.objects.create(title="Sense and Sensibility")
+
+        self.author1 = Author2.objects.create(name="Charlotte",
+                                             first_book=self.book1)
+        self.author2 = Author2.objects.create(name="Anne",
+                                             first_book=self.book1)
+        self.author3 = Author2.objects.create(name="Emily",
+                                             first_book=self.book1)
+        self.author4 = Author2.objects.create(name="Jane",
+                                             first_book=self.book4)
+
+        self.author1.favorite_books.add(self.book1, self.book2, self.book3)
+        self.author2.favorite_books.add(self.book1)
+        self.author3.favorite_books.add(self.book2)
+        self.author4.favorite_books.add(self.book3)
+
+    def test_bug(self):
+        list(Author2.objects.prefetch_related('first_book', 'favorite_books'))