Option 1

django/db/models/sql/query.py

@@ -845,8 +845,8 @@ def join(self, connection, reuse=None, outer_if_first=False,
         (matching the connection) are reusable, or it can be a set containing
         the aliases that can be reused.
 
-        If 'outer_if_first' is True and a new join is created, it will have the
-        LOUTER join type.
+        If 'outer_if_first' is True and a new join is created (or a join with
+        refcount 0 is reused), it will have the LOUTER join type.
 
         A join is always created as LOUTER if the lhs alias is LOUTER to make
         sure we do not generate chains like t1 LOUTER t2 INNER t3.
@@ -870,6 +870,8 @@ def join(self, connection, reuse=None, outer_if_first=False,
                 # check that the existing join is created using the same
                 # join_field used for the under work join.
                 continue
+            if self.alias_refcount[alias] == 0 and outer_if_first:
+                self.promote_joins([alias])
             self.ref_alias(alias)
             return alias
 

tests/select_related_regress/models.py

@@ -95,6 +95,18 @@ class Item(models.Model):
     def __str__(self):
         return self.name
 
+class ChildA(models.Model):
+    parent = models.ForeignKey(Parent)
+
+@python_2_unicode_compatible
+class ChildB(models.Model):
+    name = models.CharField(max_length=10)
+    parent = models.ForeignKey(Parent, null=True)
+    child = models.ForeignKey(ChildA, null=True)
+
+    def __str__(self):
+        return self.name
+
 # Models for testing bug #19870.
 @python_2_unicode_compatible
 class Fowl(models.Model):

tests/select_related_regress/tests.py

@@ -1,11 +1,12 @@
 from __future__ import absolute_import, unicode_literals
 
+from django.db import models
 from django.test import TestCase
 from django.utils import six
 
 from .models import (Building, Child, Device, Port, Item, Country, Connection,
     ClientStatus, State, Client, SpecialClient, TUser, Person, Student,
-    Organizer, Class, Enrollment, Hen, Chick)
+    Organizer, Class, Enrollment, Hen, Chick, Parent, ChildA, ChildB)
 
 
 class SelectRelatedRegressTests(TestCase):
@@ -173,3 +174,20 @@ def test_regression_19870(self):
 
         self.assertEqual(Chick.objects.all()[0].mother.name, 'Hen')
         self.assertEqual(Chick.objects.select_related()[0].mother.name, 'Hen')
+
+    def test_ticket_20528(self):
+        """
+        Regression for #20528
+
+        Using Qs and select_related with a certain config of ForeignKeys
+        resulted in incorrect joins.
+        """
+        parent = Parent.objects.create(name="foo")
+        childA = ChildA.objects.create(parent=parent)
+        childB = ChildB.objects.create(child=childA, name="Bobby")
+        queryset = ChildB.objects.filter(
+            models.Q(parent=parent) | models.Q(child__parent=parent)
+        )
+        self.assertQuerysetEqual(queryset, [childB], lambda x: x)
+        queryset = queryset.select_related('parent')
+        self.assertQuerysetEqual(queryset, [childB], lambda x: x)