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.

django/db/models/query.py

@@ -1296,7 +1296,7 @@ def get_klass_info(klass, max_depth=0, cur_depth=0, requested=None,
         # Build the list of fields that *haven't* been requested
         for field, model in klass._meta.get_fields_with_model():
             if field.name not in load_fields:
-                skip.add(field.name)
+                skip.add(field.attname)
             elif local_only and model is not None:
                 continue
             else:
@@ -1327,7 +1327,7 @@ def get_klass_info(klass, max_depth=0, cur_depth=0, requested=None,
 
     related_fields = []
     for f in klass._meta.fields:
-        if select_related_descend(f, restricted, requested):
+        if select_related_descend(f, restricted, requested, load_fields):
             if restricted:
                 next = requested[f.name]
             else:
@@ -1339,7 +1339,8 @@ def get_klass_info(klass, max_depth=0, cur_depth=0, requested=None,
     reverse_related_fields = []
     if restricted:
         for o in klass._meta.get_all_related_objects():
-            if o.field.unique and select_related_descend(o.field, restricted, requested, reverse=True):
+            if o.field.unique and select_related_descend(o.field, restricted, requested,
+                                                         only_load.get(o.model), reverse=True):
                 next = requested[o.field.related_query_name()]
                 klass_info = get_klass_info(o.model, max_depth=max_depth, cur_depth=cur_depth+1,
                                             requested=next, only_load=only_load, local_only=True)

django/db/models/query_utils.py

@@ -126,18 +126,19 @@ class DeferredAttribute(object):
         return None
 
 
-def select_related_descend(field, restricted, requested, reverse=False):
+def select_related_descend(field, restricted, requested, load_fields, reverse=False):
     """
     Returns True if this field should be used to descend deeper for
     select_related() purposes. Used by both the query construction code
     (sql.query.fill_related_selections()) and the model instance creation code
-    (query.get_cached_row()).
+    (query.get_klass_info()).
 
     Arguments:
      * field - the field to be checked
      * restricted - a boolean field, indicating if the field list has been
        manually restricted using a requested clause)
      * requested - The select_related() dictionary.
+     * load_fields - the set of fields to be loaded on this model
      * reverse - boolean, True if we are checking a reverse select related
     """
     if not field.rel:
@@ -151,6 +152,14 @@ def select_related_descend(field, restricted, requested, reverse=False):
             return False
     if not restricted and field.null:
         return False
+    if load_fields:
+        if field.name not in load_fields:
+            if restricted and field.name in requested:
+                raise InvalidQuery("Field %s.%s cannot be both deferred"
+                                   " and traversed using select_related"
+                                   " at the same time." %
+                                   (field.model._meta.object_name, field.name))
+            return False
     return True
 
 # This function is needed because data descriptors must be defined on a class

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

django/db/models/sql/query.py

@@ -1845,8 +1845,14 @@ class Query(object):
 
         If no fields are marked for deferral, returns an empty dictionary.
         """
+        # 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):

docs/ref/models/querysets.txt

@@ -1081,11 +1081,13 @@ to ``defer()``::
     # Load all fields immediately.
     my_queryset.defer(None)
 
+.. versionchanged:: 1.5
+
 Some fields in a model won't be deferred, even if you ask for them. You can
 never defer the loading of the primary key. If you are using
 :meth:`select_related()` to retrieve related models, you shouldn't defer the
-loading of the field that connects from the primary model to the related one
+loading of the field that connects from the primary model to the related
-(at the moment, that doesn't raise an error, but it will eventually).
+one, doing so will result in an error.
 
 .. note::
 
@@ -1145,9 +1147,12 @@ logically::
     # existing set of fields).
     Entry.objects.defer("body").only("headline", "body")
 
+.. versionchanged:: 1.5
+
 All of the cautions in the note for the :meth:`defer` documentation apply to
 ``only()`` as well. Use it cautiously and only after exhausting your other
-options.
+options. Also note that using :meth:`only` and omitting a field requested
+using :meth:`select_related` is an error as well.
 
 using
 ~~~~~

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?
+        # When we defer a field and also select_related it, the query is
-        self.assert_delayed(qs.only("name").select_related("related")[0], 1)
+        # invalid and raises an exception.
-        self.assert_delayed(qs.defer("related").select_related("related")[0], 0)
+        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')
 

tests/regressiontests/defer_regress/models.py

@@ -47,3 +47,7 @@ class SimpleItem(models.Model):
 
 class Feature(models.Model):
     item = models.ForeignKey(SimpleItem)
+
+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)
+    SimpleItem, Feature, ItemAndSimpleItem)
 
 
 class DeferRegressionTest(TestCase):
@@ -109,6 +109,7 @@ class DeferRegressionTest(TestCase):
                 Child,
                 Feature,
                 Item,
+                ItemAndSimpleItem,
                 Leaf,
                 Proxy,
                 RelatedItem,
@@ -125,12 +126,16 @@ class DeferRegressionTest(TestCase):
                 ),
             )
         )
+        # FIXME: This is dependent on the order in which tests are run --
+        # this test case has to be the first, otherwise a LOT more classes
+        # appear.
         self.assertEqual(
             klasses, [
                 "Child",
                 "Child_Deferred_value",
                 "Feature",
                 "Item",
+                "ItemAndSimpleItem",
                 "Item_Deferred_name",
                 "Item_Deferred_name_other_value_text",
                 "Item_Deferred_name_other_value_value",
@@ -139,7 +144,7 @@ class DeferRegressionTest(TestCase):
                 "Leaf",
                 "Leaf_Deferred_child_id_second_child_id_value",
                 "Leaf_Deferred_name_value",
-                "Leaf_Deferred_second_child_value",
+                "Leaf_Deferred_second_child_id_value",
                 "Leaf_Deferred_value",
                 "Proxy",
                 "RelatedItem",
@@ -175,6 +180,23 @@ class DeferRegressionTest(TestCase):
         self.assertEqual(1, qs.count())
         self.assertEqual('Foobar', qs[0].name)
 
+    def test_defer_with_select_related(self):
+        item1 = Item.objects.create(name="first", value=47)
+        item2 = Item.objects.create(name="second", value=42)
+        simple = SimpleItem.objects.create(name="simple", value="23")
+        related = ItemAndSimpleItem.objects.create(item=item1, simple=simple)
+
+        obj = ItemAndSimpleItem.objects.defer('item').select_related('simple').get()
+        self.assertEqual(obj.item, item1)
+        self.assertEqual(obj.item_id, item1.id)
+
+        obj.item = item2
+        obj.save()
+
+        obj = ItemAndSimpleItem.objects.defer('item').select_related('simple').get()
+        self.assertEqual(obj.item, item2)
+        self.assertEqual(obj.item_id, item2.id)
+
     def test_deferred_class_factory(self):
         from django.db.models.query_utils import deferred_class_factory
         new_class = deferred_class_factory(Item,

tests/regressiontests/select_related_regress/tests.py

@@ -133,7 +133,7 @@ class SelectRelatedRegressTests(TestCase):
         self.assertEqual(troy.state.name, 'Western Australia')
 
         # Also works if you use only, rather than defer
-        troy = SpecialClient.objects.select_related('state').only('name').get(name='Troy Buswell')
+        troy = SpecialClient.objects.select_related('state').only('name', 'state').get(name='Troy Buswell')
 
         self.assertEqual(troy.name, 'Troy Buswell')
         self.assertEqual(troy.value, 42)