[1.5.x] Fixed prefetch_related + pickle regressions

There were a couple of regressions related to field pickling. The
regressions were introduced by QuerySet._known_related_objects caching.

The regressions aren't present in master, the fix was likely in
f403653cf1.

Fixed #20157, fixed #20257. Also made QuerySets with model=None
picklable.

tests/regressiontests/queryset_pickle/tests.py

@@ -5,7 +5,8 @@ import datetime
 
 from django.test import TestCase
 
-from .models import Group, Event, Happening
+from .models import Group, Event, Happening, Person, Post
+from django.contrib.auth.models import AnonymousUser
 
 
 class PickleabilityTestCase(TestCase):
@@ -46,3 +47,29 @@ class PickleabilityTestCase(TestCase):
         # can't just use assertEqual(original, unpickled)
         self.assertEqual(original.__class__, unpickled.__class__)
         self.assertEqual(original.args, unpickled.args)
+
+    def test_pickle_m2m_prefetch_related(self):
+        bob = Person(name="Bob")
+        bob.save()
+        people = Person.objects.prefetch_related('socialprofile_set')
+        dumped = pickle.dumps(people)
+        people = pickle.loads(dumped)
+        self.assertQuerysetEqual(
+            people, [bob], lambda x: x)
+
+    def test_pickle_field_default_prefetch_related(self):
+        p1 = Post.objects.create()
+        posts = Post.objects.prefetch_related('materials')
+        dumped = pickle.dumps(posts)
+        posts = pickle.loads(dumped)
+        self.assertQuerysetEqual(
+            posts, [p1], lambda x: x)
+
+    def test_pickle_emptyqs(self):
+        u = AnonymousUser()
+        # Use AnonymousUser, as AnonymousUser.groups has qs.model = None
+        empty = u.groups.all()
+        dumped = pickle.dumps(empty)
+        empty = pickle.loads(dumped)
+        self.assertQuerysetEqual(
+            empty, [])