[1.5.x] Fixed #19607 - prefetch_related crash

Thanks to av@rdf.ru and flarno11@yahoo.de for the report.

Backport of 4fd94969d8 from master

django/db/models/query.py

@@ -1726,8 +1726,13 @@ def prefetch_related_objects(result_cache, related_lookups):
                         good_objects = False
                         break
                 else:
-                    # We already did this list
-                    break
+                    # Since prefetching can re-use instances, it is possible to
+                    # have the same instance multiple times in obj_list. So we
+                    # can reach this branch either because we did all of
+                    # obj_list already, or because we did 'obj' earlier in this
+                    # iteration over obj_list. In the first case we could
+                    # shortcut and exit the loop, but not in the second.
+                    continue
             if not good_objects:
                 break
 

tests/modeltests/prefetch_related/models.py

@@ -195,3 +195,23 @@ class Employee(models.Model):
 
     class Meta:
         ordering = ['id']
+
+
+### Ticket 19607
+
+@python_2_unicode_compatible
+class LessonEntry(models.Model):
+    name1 = models.CharField(max_length=200)
+    name2 = models.CharField(max_length=200)
+
+    def __str__(self):
+        return "%s %s" % (self.name1, self.name2)
+
+
+@python_2_unicode_compatible
+class WordEntry(models.Model):
+    lesson_entry = models.ForeignKey(LessonEntry)
+    name = models.CharField(max_length=200)
+
+    def __str__(self):
+        return "%s (%s)" % (self.name, self.id)

tests/modeltests/prefetch_related/tests.py

@@ -8,7 +8,8 @@ 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)
+    BookWithYear, BookReview, Person, House, Room, Employee, Comment,
+    LessonEntry, WordEntry)
 
 
 class PrefetchRelatedTests(TestCase):
@@ -618,3 +619,25 @@ class MultiDbTests(TestCase):
             ages = ", ".join(str(a.authorwithage.age) for a in A.prefetch_related('authorwithage'))
 
         self.assertEqual(ages, "50, 49")
+
+
+class Ticket19607Tests(TestCase):
+
+    def setUp(self):
+
+        for id, name1, name2 in [
+            (1, 'einfach', 'simple'),
+            (2, 'schwierig', 'difficult'),
+            ]:
+            LessonEntry.objects.create(id=id, name1=name1, name2=name2)
+
+        for id, lesson_entry_id, name in [
+            (1, 1, 'einfach'),
+            (2, 1, 'simple'),
+            (3, 2, 'schwierig'),
+            (4, 2, 'difficult'),
+            ]:
+            WordEntry.objects.create(id=id, lesson_entry_id=lesson_entry_id, name=name)
+
+    def test_bug(self):
+        list(WordEntry.objects.prefetch_related('lesson_entry', 'lesson_entry__wordentry_set'))