[1.11.x] Fixed #26522 -- Fixed a nondeterministic AssertionError in QuerySet combining.

Thanks Andrew Brown for the test case.

Backport of 9bbb6e2d2536c4ac20dc13a94c1f80494e51f8d9 from master

AUTHORS

@@ -110,6 +110,7 @@ answer newbie questions, and generally made Django that much better:
     berto
     Bill Fenner <fenner@gmail.com>
     Bjørn Stabell <bjorn@exoweb.net>
+    Bo Marchman <bo.marchman@gmail.com>
     Bojan Mihelac <bmihelac@mihelac.org>
     Bouke Haarsma <bouke@haarsma.eu>
     Božidar Benko <bbenko@gmail.com>

django/db/models/sql/query.py

@@ -133,7 +133,7 @@ class Query(object):
         # types they are. The key is the alias of the joined table (possibly
         # the table name) and the value is a Join-like object (see
         # sql.datastructures.Join for more information).
-        self.alias_map = {}
+        self.alias_map = OrderedDict()
         # Sometimes the query contains references to aliases in outer queries (as
         # a result of split_exclude). Correct alias quoting needs to know these
         # aliases too.

docs/releases/1.11.5.txt

@@ -40,3 +40,6 @@ Bugfixes
   weren't logged in the admin change history (:ticket:`27998`) and prevented
   ``ManyToManyField`` initial data in model forms from being affected by
   subsequent model changes (:ticket:`28543`).
+
+* Fixed non-deterministic results or an ``AssertionError`` crash in some
+  queries with multiple joins (:ticket:`26522`).

tests/queries/models.py

@@ -709,6 +709,11 @@ class Classroom(models.Model):
     students = models.ManyToManyField(Student, related_name='classroom')
 
 
+class Teacher(models.Model):
+    schools = models.ManyToManyField(School)
+    friends = models.ManyToManyField('self')
+
+
 class Ticket23605AParent(models.Model):
     pass
 

tests/queries/tests.py

@@ -28,9 +28,9 @@ from .models import (
     ProxyCategory, ProxyObjectA, ProxyObjectB, Ranking, Related,
     RelatedIndividual, RelatedObject, Report, ReportComment, ReservedName,
     Responsibility, School, SharedConnection, SimpleCategory, SingleObject,
-    SpecialCategory, Staff, StaffUser, Student, Tag, Task, Ticket21203Child,
-    Ticket21203Parent, Ticket23605A, Ticket23605B, Ticket23605C, TvChef, Valid,
-    X,
+    SpecialCategory, Staff, StaffUser, Student, Tag, Task, Teacher,
+    Ticket21203Child, Ticket21203Parent, Ticket23605A, Ticket23605B,
+    Ticket23605C, TvChef, Valid, X,
 )
 
 
@@ -1391,6 +1391,18 @@ class Queries4Tests(TestCase):
         self.assertEqual(len(combined), 1)
         self.assertEqual(combined[0].name, 'a1')
 
+    def test_join_reuse_order(self):
+        # Join aliases are reused in order. This shouldn't raise AssertionError
+        # because change_map contains a circular reference (#26522).
+        s1 = School.objects.create()
+        s2 = School.objects.create()
+        s3 = School.objects.create()
+        t1 = Teacher.objects.create()
+        otherteachers = Teacher.objects.exclude(pk=t1.pk).exclude(friends=t1)
+        qs1 = otherteachers.filter(schools=s1).filter(schools=s2)
+        qs2 = otherteachers.filter(schools=s1).filter(schools=s3)
+        self.assertQuerysetEqual(qs1 | qs2, [])
+
     def test_ticket7095(self):
         # Updates that are filtered on the model being updated are somewhat
         # tricky in MySQL.