Fixed #22050 -- Fixed defer fields on proxy related models.

django/db/models/sql/query.py

@@ -585,7 +585,7 @@ class Query(object):
         must_include = {orig_opts.concrete_model: set([orig_opts.pk])}
         for field_name in field_names:
             parts = field_name.split(LOOKUP_SEP)
-            cur_model = self.model
+            cur_model = self.model._meta.concrete_model
             opts = orig_opts
             for name in parts[:-1]:
                 old_model = cur_model

tests/defer_regress/models.py

@@ -21,6 +21,11 @@ class RelatedItem(models.Model):
     item = models.ForeignKey(Item)
 
 
+class ProxyRelated(RelatedItem):
+    class Meta:
+        proxy = True
+
+
 class Child(models.Model):
     name = models.CharField(max_length=10)
     value = models.IntegerField()

tests/defer_regress/tests.py

@@ -10,7 +10,9 @@ from django.test import TestCase, override_settings
 
 from .models import (
     ResolveThis, Item, RelatedItem, Child, Leaf, Proxy, SimpleItem, Feature,
-    ItemAndSimpleItem, OneToOneItem, SpecialFeature, Location, Request)
+    ItemAndSimpleItem, OneToOneItem, SpecialFeature, Location, Request,
+    ProxyRelated,
+)
 
 
 class DeferRegressionTest(TestCase):
@@ -207,6 +209,17 @@ class DeferRegressionTest(TestCase):
         self.assertEqual(obj.item, item2)
         self.assertEqual(obj.item_id, item2.id)
 
+    def test_proxy_model_defer_with_selected_related(self):
+        # Regression for #22050
+        item = Item.objects.create(name="first", value=47)
+        related = RelatedItem.objects.create(item=item)
+        # Defer fields with only()
+        obj = ProxyRelated.objects.all().select_related().only('item__name')[0]
+        with self.assertNumQueries(0):
+            self.assertEqual(obj.item.name, "first")
+        with self.assertNumQueries(1):
+            self.assertEqual(obj.item.value, 47)
+
     def test_only_with_select_related(self):
         # Test for #17485.
         item = SimpleItem.objects.create(name='first', value=47)