Fixed #17485 regression -- only + select_related interaction

When doing deeper than one level select_related() + only queries(), the
code introduced in b6c356b7bb errored
incorrectly.

Thanks to mrmachine for report & test case.

django/db/models/sql/compiler.py

@@ -609,8 +609,12 @@ class SQLCompiler(object):
                 restricted = False
 
         for f, model in opts.get_fields_with_model():
+            # The get_fields_with_model() returns None for fields that live
+            # in the field's local model. So, for those fields we want to use
+            # the f.model - that is the field's local model.
+            field_model = model or f.model
             if not select_related_descend(f, restricted, requested,
-                                          only_load.get(model or self.query.model)):
+                                          only_load.get(field_model)):
                 continue
             # The "avoid" set is aliases we want to avoid just for this
             # particular branch of the recursion. They aren't permanently

tests/regressiontests/defer_regress/models.py

@@ -52,6 +52,9 @@ class SimpleItem(models.Model):
 class Feature(models.Model):
     item = models.ForeignKey(SimpleItem)
 
+class SpecialFeature(models.Model):
+    feature = models.ForeignKey(Feature)
+
 class ItemAndSimpleItem(models.Model):
     item = models.ForeignKey(Item)
     simple = models.ForeignKey(SimpleItem)

tests/regressiontests/defer_regress/tests.py

@@ -9,7 +9,7 @@ from django.db.models.loading import cache
 from django.test import TestCase
 
 from .models import (ResolveThis, Item, RelatedItem, Child, Leaf, Proxy,
-    SimpleItem, Feature, ItemAndSimpleItem)
+    SimpleItem, Feature, ItemAndSimpleItem, SpecialFeature)
 
 
 class DeferRegressionTest(TestCase):
@@ -115,6 +115,7 @@ class DeferRegressionTest(TestCase):
                 RelatedItem,
                 ResolveThis,
                 SimpleItem,
+                SpecialFeature,
             ]
         )
 
@@ -152,6 +153,7 @@ class DeferRegressionTest(TestCase):
                 "RelatedItem_Deferred_item_id",
                 "ResolveThis",
                 "SimpleItem",
+                "SpecialFeature",
             ]
         )
 
@@ -197,6 +199,18 @@ class DeferRegressionTest(TestCase):
         self.assertEqual(obj.item, item2)
         self.assertEqual(obj.item_id, item2.id)
 
+    def test_only_with_select_related(self):
+        # Test for #17485.
+        item = SimpleItem.objects.create(name='first', value=47)
+        feature = Feature.objects.create(item=item)
+        SpecialFeature.objects.create(feature=feature)
+
+        qs = Feature.objects.only('item__name').select_related('item')
+        self.assertEqual(len(qs), 1)
+
+        qs = SpecialFeature.objects.only('feature__item__name').select_related('feature__item')
+        self.assertEqual(len(qs), 1)
+
     def test_deferred_class_factory(self):
         from django.db.models.query_utils import deferred_class_factory
         new_class = deferred_class_factory(Item,