Fixed #27594 -- Fixed select_related() with reverse self-referential OneToOneField.

Fix definition of `klass_info['from_parent']`. The relationship between
two models shouldn't be considered as being from a parent class if the
model classes are the same.

Thanks Tim for the review.

django/db/models/sql/compiler.py

@@ -724,7 +724,7 @@ class SQLCompiler(object):
 
                 _, _, _, joins, _ = self.query.setup_joins([related_field_name], opts, root_alias)
                 alias = joins[-1]
-                from_parent = issubclass(model, opts.model)
+                from_parent = issubclass(model, opts.model) and model is not opts.model
                 klass_info = {
                     'model': model,
                     'field': f,

tests/select_related_onetoone/models.py

@@ -102,3 +102,12 @@ class Child3(Child2):
 
 class Child4(Child1):
     value4 = models.IntegerField()
+
+
+class LinkedList(models.Model):
+    name = models.CharField(max_length=50)
+    previous_item = models.OneToOneField(
+        'self', models.CASCADE,
+        related_name="next_item", blank=True,
+        null=True,
+    )

tests/select_related_onetoone/tests.py

@@ -6,8 +6,9 @@ from django.core.exceptions import FieldError
 from django.test import TestCase
 
 from .models import (
-    AdvancedUserStat, Child1, Child2, Child3, Child4, Image, Parent1, Parent2,
+    AdvancedUserStat, Child1, Child2, Child3, Child4, Image, LinkedList,
-    Product, StatDetails, User, UserProfile, UserStat, UserStatResult,
+    Parent1, Parent2, Product, StatDetails, User, UserProfile, UserStat,
+    UserStatResult,
 )
 
 
@@ -212,6 +213,13 @@ class ReverseSelectRelatedTestCase(TestCase):
         with self.assertNumQueries(1):
             self.assertEqual(p.child1.child4.name1, 'n1')
 
+    def test_self_relation(self):
+        item1 = LinkedList.objects.create(name='item1')
+        LinkedList.objects.create(name='item2', previous_item=item1)
+        with self.assertNumQueries(1):
+            item1_db = LinkedList.objects.select_related('next_item').get(name='item1')
+            self.assertEqual(item1_db.next_item.name, 'item2')
+
 
 class ReverseSelectRelatedValidationTests(TestCase):
     """