Fixed #20242 - Added a regression test for prefetch_related.

Issue was fixed by removal of chunked reads from
QuerySet iteration in 70679243d1.

Thanks Simeon Visser for the patch.

AUTHORS

@@ -594,6 +594,7 @@ answer newbie questions, and generally made Django that much better:
     I.S. van Oostveen <v.oostveen@idca.nl>
     viestards.lists@gmail.com
     George Vilches <gav@thataddress.com>
+    Simeon Visser <http://simeonvisser.com>
     Vlado <vlado@labath.org>
     Zachary Voase <zacharyvoase@gmail.com>
     Marijn Vriens <marijn@metronomo.cl>

tests/prefetch_related/tests.py

@@ -107,6 +107,16 @@ class PrefetchRelatedTests(TestCase):
             qs = Book.objects.prefetch_related('first_time_authors')
             [b.first_time_authors.exists() for b in qs]
 
+    def test_in_and_prefetch_related(self):
+        """
+        Regression test for #20242 - QuerySet "in" didn't work the first time
+        when using prefetch_related. This was fixed by the removal of chunked
+        reads from QuerySet iteration in
+        70679243d1786e03557c28929f9762a119e3ac14.
+        """
+        qs = Book.objects.prefetch_related('first_time_authors')
+        self.assertTrue(qs[0] in qs)
+
     def test_clear(self):
         """
         Test that we can clear the behavior by calling prefetch_related()