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.

2 files changed, 14 insertions, 5 deletions

diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 5801b2f428..d44cdfe4a4 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -596,6 +596,7 @@ class SQLCompiler(object):
         if avoid_set is None:
             avoid_set = set()
         orig_dupe_set = dupe_set
+        only_load = self.query.get_loaded_field_names()
 
         # Setup for the case when only particular related fields should be
         # included in the related selection.
@@ -607,7 +608,8 @@ class SQLCompiler(object):
                 restricted = False
 
         for f, model in opts.get_fields_with_model():
-            if not select_related_descend(f, restricted, requested):
+            if not select_related_descend(f, restricted, requested,
+                                          only_load.get(model or self.query.model)):
                 continue
             # The "avoid" set is aliases we want to avoid just for this
             # particular branch of the recursion. They aren't permanently
@@ -680,7 +682,8 @@ class SQLCompiler(object):
                 if o.field.unique
             ]
             for f, model in related_fields:
-                if not select_related_descend(f, restricted, requested, reverse=True):
+                if not select_related_descend(f, restricted, requested,
+                                              only_load.get(model), reverse=True):
                     continue
                 # The "avoid" set is aliases we want to avoid just for this
                 # particular branch of the recursion. They aren't permanently
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 7f331bfe7f..8fbba3dbc9 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -1845,9 +1845,15 @@ class Query(object):
 
         If no fields are marked for deferral, returns an empty dictionary.
         """
-        collection = {}
-        self.deferred_to_data(collection, self.get_loaded_field_names_cb)
-        return collection
+        # We cache this because we call this function multiple times
+        # (compiler.fill_related_selections, query.iterator)
+        try:
+            return self._loaded_field_names_cache
+        except AttributeError:
+            collection = {}
+            self.deferred_to_data(collection, self.get_loaded_field_names_cb)
+            self._loaded_field_names_cache = collection
+            return collection
 
     def get_loaded_field_names_cb(self, target, model, fields):
         """