Fixed #30727 -- Made Subquery pickle without evaluating their QuerySet.

Subquery expression objects, when pickled, were evaluating the QuerySet
objects saved in its _constructor_args attribute.

django/db/models/expressions.py

@@ -1019,6 +1019,11 @@ class Subquery(Expression):
         self.extra = extra
         super().__init__(output_field)
 
+    def __getstate__(self):
+        state = super().__getstate__()
+        state.pop('_constructor_args', None)
+        return state
+
     def get_source_expressions(self):
         return [self.query]
 

tests/queryset_pickle/tests.py

@@ -172,6 +172,41 @@ class PickleabilityTestCase(TestCase):
         m2ms = pickle.loads(pickle.dumps(m2ms))
         self.assertSequenceEqual(m2ms, [m2m])
 
+    def test_pickle_exists_queryset_still_usable(self):
+        group = Group.objects.create(name='group')
+        Event.objects.create(title='event', group=group)
+        groups = Group.objects.annotate(
+            has_event=models.Exists(
+                Event.objects.filter(group_id=models.OuterRef('id')),
+            ),
+        )
+        groups2 = pickle.loads(pickle.dumps(groups))
+        self.assertSequenceEqual(groups2.filter(has_event=True), [group])
+
+    def test_pickle_exists_queryset_not_evaluated(self):
+        group = Group.objects.create(name='group')
+        Event.objects.create(title='event', group=group)
+        groups = Group.objects.annotate(
+            has_event=models.Exists(
+                Event.objects.filter(group_id=models.OuterRef('id')),
+            ),
+        )
+        list(groups)  # evaluate QuerySet.
+        with self.assertNumQueries(0):
+            self.assert_pickles(groups)
+
+    def test_pickle_subquery_queryset_not_evaluated(self):
+        group = Group.objects.create(name='group')
+        Event.objects.create(title='event', group=group)
+        groups = Group.objects.annotate(
+            event_title=models.Subquery(
+                Event.objects.filter(group_id=models.OuterRef('id')).values('title'),
+            ),
+        )
+        list(groups)  # evaluate QuerySet.
+        with self.assertNumQueries(0):
+            self.assert_pickles(groups)
+
     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'))