Fixed #17485 -- Made defer work with select_related

This commit tackles a couple of issues. First, in certain cases there
were some mixups if field.attname or field.name should be deferred.
Field.attname is now always used.

Another issue tackled is a case where field is both deferred by
.only(), and selected by select_related. This case is now an error.

A lot of thanks to koniiiik (Michal Petrucha) for the patch, and
to Andrei Antoukh for review.

1 files changed, 15 insertions, 5 deletions

diff --git a/tests/modeltests/defer/tests.py b/tests/modeltests/defer/tests.py
index eb09162b01..50db5a76b4 100644
--- a/tests/modeltests/defer/tests.py
+++ b/tests/modeltests/defer/tests.py
@@ -1,6 +1,6 @@
 from __future__ import absolute_import
 
-from django.db.models.query_utils import DeferredAttribute
+from django.db.models.query_utils import DeferredAttribute, InvalidQuery
 from django.test import TestCase
 
 from .models import Secondary, Primary, Child, BigChild, ChildProxy
@@ -73,9 +73,19 @@ class DeferTests(TestCase):
         self.assert_delayed(qs.defer("name").get(pk=p1.pk), 1)
         self.assert_delayed(qs.only("name").get(pk=p1.pk), 2)
 
-        # DOES THIS WORK?
-        self.assert_delayed(qs.only("name").select_related("related")[0], 1)
-        self.assert_delayed(qs.defer("related").select_related("related")[0], 0)
+        # When we defer a field and also select_related it, the query is
+        # invalid and raises an exception.
+        with self.assertRaises(InvalidQuery):
+            qs.only("name").select_related("related")[0]
+        with self.assertRaises(InvalidQuery):
+            qs.defer("related").select_related("related")[0]
+
+        # With a depth-based select_related, all deferred ForeignKeys are
+        # deferred instead of traversed.
+        with self.assertNumQueries(3):
+            obj = qs.defer("related").select_related()[0]
+            self.assert_delayed(obj, 1)
+            self.assertEqual(obj.related.id, s1.pk)
 
         # Saving models with deferred fields is possible (but inefficient,
         # since every field has to be retrieved first).
@@ -155,7 +165,7 @@ class DeferTests(TestCase):
         children = ChildProxy.objects.all().select_related().only('id', 'name')
         self.assertEqual(len(children), 1)
         child = children[0]
-        self.assert_delayed(child, 1)
+        self.assert_delayed(child, 2)
         self.assertEqual(child.name, 'p1')
         self.assertEqual(child.value, 'xx')