Refs #28586 -- Made fetch modes pickle as singletons.

This change ensures that we don’t create new instances of fetch modes
when pickling and unpickling, saving memory and preserving their singleton
nature.

django/db/models/fetch_modes.py

@@ -16,6 +16,9 @@ class FetchOne(FetchMode):
     def fetch(self, fetcher, instance):
         fetcher.fetch_one(instance)
 
+    def __reduce__(self):
+        return "FETCH_ONE"
+
 
 FETCH_ONE = FetchOne()
 
@@ -36,6 +39,9 @@ class FetchPeers(FetchMode):
         else:
             fetcher.fetch_one(instance)
 
+    def __reduce__(self):
+        return "FETCH_PEERS"
+
 
 FETCH_PEERS = FetchPeers()
 
@@ -48,5 +54,8 @@ class Raise(FetchMode):
         field_name = fetcher.field.name
         raise FieldFetchBlocked(f"Fetching of {klass}.{field_name} blocked.") from None
 
+    def __reduce__(self):
+        return "RAISE"
+
 
 RAISE = Raise()

tests/queryset_pickle/tests.py

@@ -351,6 +351,29 @@ class PickleabilityTestCase(TestCase):
         event.edition_set.create()
         self.assert_pickles(event.edition_set.order_by("event"))
 
+    def test_fetch_mode_fetch_one(self):
+        restored = pickle.loads(pickle.dumps(self.happening))
+        self.assertIs(restored._state.fetch_mode, models.FETCH_ONE)
+
+    def test_fetch_mode_fetch_peers(self):
+        Happening.objects.create()
+        objs = list(Happening.objects.fetch_mode(models.FETCH_PEERS))
+        self.assertEqual(objs[0]._state.fetch_mode, models.FETCH_PEERS)
+        self.assertEqual(len(objs[0]._state.peers), 2)
+
+        restored = pickle.loads(pickle.dumps(objs))
+
+        self.assertIs(restored[0]._state.fetch_mode, models.FETCH_PEERS)
+        # Peers not restored because weak references are not picklable.
+        self.assertEqual(restored[0]._state.peers, ())
+
+    def test_fetch_mode_raise(self):
+        objs = list(Happening.objects.fetch_mode(models.RAISE))
+        self.assertEqual(objs[0]._state.fetch_mode, models.RAISE)
+
+        restored = pickle.loads(pickle.dumps(objs))
+        self.assertIs(restored[0]._state.fetch_mode, models.RAISE)
+
 
 class InLookupTests(TestCase):
     @classmethod