Fixed #31926 -- Fixed recreating queryset with FilteredRelation when using a pickled Query.

In a pickled join, the join_fields had the same values, but weren't the
same object (contrary to when not pickling the QuerySet).

django/db/models/fields/reverse_related.py

@@ -11,6 +11,7 @@ they're the closest concept currently available.
 
 from django.core import exceptions
 from django.utils.functional import cached_property
+from django.utils.hashable import make_hashable
 
 from . import BLANK_CHOICE_DASH
 from .mixins import FieldCacheMixin
@@ -115,6 +116,28 @@ class ForeignObjectRel(FieldCacheMixin):
             self.related_model._meta.model_name,
         )
 
+    @property
+    def identity(self):
+        return (
+            self.field,
+            self.model,
+            self.related_name,
+            self.related_query_name,
+            tuple(sorted(make_hashable(self.limit_choices_to))),
+            self.parent_link,
+            self.on_delete,
+            self.symmetrical,
+            self.multiple,
+        )
+
+    def __eq__(self, other):
+        if not isinstance(other, self.__class__):
+            return NotImplemented
+        return self.identity == other.identity
+
+    def __hash__(self):
+        return hash(self.identity)
+
     def get_choices(
         self, include_blank=True, blank_choice=BLANK_CHOICE_DASH,
         limit_choices_to=None, ordering=(),
@@ -215,6 +238,10 @@ class ManyToOneRel(ForeignObjectRel):
         state.pop('related_model', None)
         return state
 
+    @property
+    def identity(self):
+        return super().identity + (self.field_name,)
+
     def get_related_field(self):
         """
         Return the Field in the 'to' object to which this relationship is tied.
@@ -279,6 +306,14 @@ class ManyToManyRel(ForeignObjectRel):
         self.symmetrical = symmetrical
         self.db_constraint = db_constraint
 
+    @property
+    def identity(self):
+        return super().identity + (
+            self.through,
+            self.through_fields,
+            self.db_constraint,
+        )
+
     def get_related_field(self):
         """
         Return the field in the 'to' object to which this relationship is tied.

tests/queryset_pickle/tests.py

@@ -219,6 +219,40 @@ class PickleabilityTestCase(TestCase):
         with self.assertNumQueries(0):
             self.assert_pickles(groups)
 
+    def test_pickle_filteredrelation(self):
+        group = Group.objects.create(name='group')
+        event_1 = Event.objects.create(title='Big event', group=group)
+        event_2 = Event.objects.create(title='Small event', group=group)
+        Happening.objects.bulk_create([
+            Happening(event=event_1, number1=5),
+            Happening(event=event_2, number1=3),
+        ])
+        groups = Group.objects.annotate(
+            big_events=models.FilteredRelation(
+                'event',
+                condition=models.Q(event__title__startswith='Big'),
+            ),
+        ).annotate(sum_number=models.Sum('big_events__happening__number1'))
+        groups_query = pickle.loads(pickle.dumps(groups.query))
+        groups = Group.objects.all()
+        groups.query = groups_query
+        self.assertEqual(groups.get().sum_number, 5)
+
+    def test_pickle_filteredrelation_m2m(self):
+        group = Group.objects.create(name='group')
+        m2mmodel = M2MModel.objects.create()
+        m2mmodel.groups.add(group)
+        groups = Group.objects.annotate(
+            first_m2mmodels=models.FilteredRelation(
+                'm2mmodel',
+                condition=models.Q(m2mmodel__pk__lt=10),
+            ),
+        ).annotate(count_groups=models.Count('first_m2mmodels__groups'))
+        groups_query = pickle.loads(pickle.dumps(groups.query))
+        groups = Group.objects.all()
+        groups.query = groups_query
+        self.assertEqual(groups.get().count_groups, 1)
+
     def test_annotation_with_callable_default(self):
         # Happening.when has a callable default of datetime.datetime.now.
         qs = Happening.objects.annotate(latest_time=models.Max('when'))