Fixed #18343 -- Cleaned up deferred model implementation

Generic cleanup and dead code removal in deferred model field loading
and model.__reduce__().

Also fixed an issue where if an inherited model with a parent field
chain parent_ptr_id -> id would be deferred loaded, then accessing
the id field caused caused a database query, even if the id field's
value is already loaded in the parent_ptr_id field.

tests/modeltests/defer/tests.py

@@ -158,3 +158,18 @@ class DeferTests(TestCase):
         self.assert_delayed(child, 1)
         self.assertEqual(child.name, 'p1')
         self.assertEqual(child.value, 'xx')
+
+    def test_defer_inheritance_pk_chaining(self):
+        """
+        When an inherited model is fetched from the DB, its PK is also fetched.
+        When getting the PK of the parent model it is useful to use the already
+        fetched parent model PK if it happens to be available. Tests that this
+        is done.
+        """
+        s1 = Secondary.objects.create(first="x1", second="y1")
+        bc = BigChild.objects.create(name="b1", value="foo", related=s1,
+                                     other="bar")
+        bc_deferred = BigChild.objects.only('name').get(pk=bc.pk)
+        with self.assertNumQueries(0):
+            bc_deferred.id
+        self.assertEqual(bc_deferred.pk, bc_deferred.id)

tests/modeltests/field_subclassing/tests.py

@@ -17,6 +17,10 @@ class CustomField(TestCase):
         self.assertTrue(isinstance(d.data, list))
         self.assertEqual(d.data, [1, 2, 3])
 
+        d = DataModel.objects.defer("data").get(pk=d.pk)
+        self.assertTrue(isinstance(d.data, list))
+        self.assertEqual(d.data, [1, 2, 3])
+        # Refetch for save
         d = DataModel.objects.defer("data").get(pk=d.pk)
         d.save()