Fixed #21204 -- Tracked field deferrals by field instead of models.

This ensures field deferral works properly when a model is involved
more than once in the same query with a distinct deferral mask.

tests/defer_regress/tests.py

@@ -246,8 +246,6 @@ class DeferRegressionTest(TestCase):
         )
         self.assertEqual(len(qs), 1)
 
-
-class DeferAnnotateSelectRelatedTest(TestCase):
     def test_defer_annotate_select_related(self):
         location = Location.objects.create()
         Request.objects.create(location=location)
@@ -276,6 +274,28 @@ class DeferAnnotateSelectRelatedTest(TestCase):
             list,
         )
 
+    def test_common_model_different_mask(self):
+        child = Child.objects.create(name="Child", value=42)
+        second_child = Child.objects.create(name="Second", value=64)
+        Leaf.objects.create(child=child, second_child=second_child)
+        with self.assertNumQueries(1):
+            leaf = (
+                Leaf.objects.select_related("child", "second_child")
+                .defer("child__name", "second_child__value")
+                .get()
+            )
+            self.assertEqual(leaf.child, child)
+            self.assertEqual(leaf.second_child, second_child)
+        self.assertEqual(leaf.child.get_deferred_fields(), {"name"})
+        self.assertEqual(leaf.second_child.get_deferred_fields(), {"value"})
+        with self.assertNumQueries(0):
+            self.assertEqual(leaf.child.value, 42)
+            self.assertEqual(leaf.second_child.name, "Second")
+        with self.assertNumQueries(1):
+            self.assertEqual(leaf.child.name, "Child")
+        with self.assertNumQueries(1):
+            self.assertEqual(leaf.second_child.value, 64)
+
 
 class DeferDeletionSignalsTests(TestCase):
     senders = [Item, Proxy]