[1.6.x] Fixed #20955 -- select_related regression

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.

Backpatch of 8d65b6082c.

Conflicts:

	django/db/models/sql/compiler.py
	tests/queries/tests.py

django/db/models/sql/compiler.py

@@ -687,8 +687,7 @@ class SQLCompiler(object):
                 # Use True here because we are looking at the _reverse_ side of
                 # the relation, which is always nullable.
                 new_nullable = True
-                table = model._meta.db_table
+                self.fill_related_selections(model._meta, alias, cur_depth + 1,
-                self.fill_related_selections(model._meta, table, cur_depth+1,
                                              next, restricted, new_nullable)
 
     def deferred_to_columns(self):

tests/queries/models.py

@@ -497,3 +497,29 @@ class OrderItem(models.Model):
 
     def __str__(self):
         return '%s' % self.pk
+
+class BaseUser(models.Model):
+    pass
+
+@python_2_unicode_compatible
+class Task(models.Model):
+    title = models.CharField(max_length=10)
+    owner = models.ForeignKey(BaseUser, related_name='owner')
+    creator = models.ForeignKey(BaseUser, related_name='creator')
+
+    def __str__(self):
+        return self.title
+
+@python_2_unicode_compatible
+class Staff(models.Model):
+    name = models.CharField(max_length=10)
+
+    def __str__(self):
+        return self.name
+
+@python_2_unicode_compatible
+class StaffUser(BaseUser):
+    staff = models.OneToOneField(Staff, related_name='user')
+
+    def __str__(self):
+        return self.staff

tests/queries/tests.py

@@ -25,7 +25,7 @@ from .models import (
     OneToOneCategory, NullableName, ProxyCategory, SingleObject, RelatedObject,
     ModelA, ModelB, ModelC, ModelD, Responsibility, Job, JobResponsibilities,
     BaseA, FK1, Identifier, Program, Channel, Page, Paragraph, Chapter, Book,
-    MyObject, Order, OrderItem)
+    MyObject, Order, OrderItem, Task, Staff, StaffUser)
 
 
 class BaseQuerysetTest(TestCase):
@@ -2943,3 +2943,23 @@ class RelatedLookupTypeTests(TestCase):
         self.assertQuerysetEqual(
             ObjectB.objects.filter(objecta__in=[wrong_type]),
             [ob], lambda x: x)
+
+class Ticket20955Tests(TestCase):
+    def test_ticket_20955(self):
+        jack = Staff.objects.create(name='jackstaff')
+        jackstaff = StaffUser.objects.create(staff=jack)
+        jill = Staff.objects.create(name='jillstaff')
+        jillstaff = StaffUser.objects.create(staff=jill)
+        task = Task.objects.create(creator=jackstaff, owner=jillstaff, title="task")
+        task_get = Task.objects.get(pk=task.pk)
+        # Load data so that assertNumQueries doesn't complain about the get
+        # version's queries.
+        task_get.creator.staffuser.staff
+        task_get.owner.staffuser.staff
+        task_select_related = Task.objects.select_related(
+            'creator__staffuser__staff', 'owner__staffuser__staff').get(pk=task.pk)
+        with self.assertNumQueries(0):
+            self.assertEqual(task_select_related.creator.staffuser.staff,
+                             task_get.creator.staffuser.staff)
+            self.assertEqual(task_select_related.owner.staffuser.staff,
+                             task_get.owner.staffuser.staff)