[1.5.x] Fixed #14694 -- Made `defer()` work with reverse relations

Reverse o2o fields are now usable with defer.

Backpatch of [6ebf115206]

django/db/models/sql/query.py

@@ -585,17 +585,22 @@ class Query(object):
             for name in parts[:-1]:
                 old_model = cur_model
                 source = opts.get_field_by_name(name)[0]
-                cur_model = source.rel.to
+                if is_reverse_o2o(source):
+                    cur_model = source.model
+                else:
+                    cur_model = source.rel.to
                 opts = cur_model._meta
                 # Even if we're "just passing through" this model, we must add
                 # both the current model's pk and the related reference field
-                # to the things we select.
-                must_include[old_model].add(source)
+                # (if it's not a reverse relation) to the things we select.
+                if not is_reverse_o2o(source):
+                    must_include[old_model].add(source)
                 add_to_dict(must_include, cur_model, opts.pk)
             field, model, _, _ = opts.get_field_by_name(parts[-1])
             if model is None:
                 model = cur_model
-            add_to_dict(seen, model, field)
+            if not is_reverse_o2o(field):
+                add_to_dict(seen, model, field)
 
         if defer:
             # We need to load all fields for each model, except those that
@@ -2032,3 +2037,10 @@ def add_to_dict(data, key, value):
         data[key].add(value)
     else:
         data[key] = set([value])
+
+def is_reverse_o2o(field):
+    """
+    A little helper to check if the given field is reverse-o2o. The field is
+    expected to be some sort of relation field or related object.
+    """
+    return not hasattr(field, 'rel') and field.field.unique

tests/regressiontests/defer_regress/models.py

@@ -55,6 +55,10 @@ class Feature(models.Model):
 class SpecialFeature(models.Model):
     feature = models.ForeignKey(Feature)
 
+class OneToOneItem(models.Model):
+    item = models.OneToOneField(Item, related_name="one_to_one_item")
+    name = models.CharField(max_length=15)
+
 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, SpecialFeature)
+    SimpleItem, Feature, ItemAndSimpleItem, OneToOneItem, SpecialFeature)
 
 
 class DeferRegressionTest(TestCase):
@@ -111,6 +111,7 @@ class DeferRegressionTest(TestCase):
                 Item,
                 ItemAndSimpleItem,
                 Leaf,
+                OneToOneItem,
                 Proxy,
                 RelatedItem,
                 ResolveThis,
@@ -147,6 +148,7 @@ class DeferRegressionTest(TestCase):
                 "Leaf_Deferred_name_value",
                 "Leaf_Deferred_second_child_id_value",
                 "Leaf_Deferred_value",
+                "OneToOneItem",
                 "Proxy",
                 "RelatedItem",
                 "RelatedItem_Deferred_",
@@ -182,6 +184,33 @@ class DeferRegressionTest(TestCase):
         self.assertEqual(1, qs.count())
         self.assertEqual('Foobar', qs[0].name)
 
+    def test_reverse_one_to_one_relations(self):
+        # Refs #14694. Test reverse relations which are known unique (reverse
+        # side has o2ofield or unique FK) - the o2o case
+        item = Item.objects.create(name="first", value=42)
+        o2o = OneToOneItem.objects.create(item=item, name="second")
+        self.assertEqual(len(Item.objects.defer('one_to_one_item__name')), 1)
+        self.assertEqual(len(Item.objects.select_related('one_to_one_item')), 1)
+        self.assertEqual(len(Item.objects.select_related(
+            'one_to_one_item').defer('one_to_one_item__name')), 1)
+        self.assertEqual(len(Item.objects.select_related('one_to_one_item').defer('value')), 1)
+        # Make sure that `only()` doesn't break when we pass in a unique relation,
+        # rather than a field on the relation.
+        self.assertEqual(len(Item.objects.only('one_to_one_item')), 1)
+        with self.assertNumQueries(1):
+            i = Item.objects.select_related('one_to_one_item')[0]
+            self.assertEquals(i.one_to_one_item.pk, o2o.pk)
+            self.assertEquals(i.one_to_one_item.name, "second")
+        with self.assertNumQueries(1):
+            i = Item.objects.select_related('one_to_one_item').defer(
+                'value', 'one_to_one_item__name')[0]
+            self.assertEquals(i.one_to_one_item.pk, o2o.pk)
+            self.assertEquals(i.name, "first")
+        with self.assertNumQueries(1):
+            self.assertEquals(i.one_to_one_item.name, "second")
+        with self.assertNumQueries(1):
+            self.assertEquals(i.value, 42)
+
     def test_defer_with_select_related(self):
         item1 = Item.objects.create(name="first", value=47)
         item2 = Item.objects.create(name="second", value=42)