Fixed #19870 -- Regression in select_related in inheritance cases

There was a regression in case two models inherited the same parent,
and one contained a foreign key to other. When select_related travelled
the foreign key the other model reused the parent join made by the
first model. This was likely caused by Query.join_parent_model()
addition in commit 68985db482.

Thanks to Trac alias loic84 for report & tests.

django/db/models/sql/compiler.py

@@ -265,14 +265,19 @@ class SQLCompiler(object):
         qn2 = self.connection.ops.quote_name
         aliases = set()
         only_load = self.deferred_to_columns()
-        seen = self.query.included_inherited_models.copy()
-        if start_alias:
-            seen[None] = start_alias
+        if not start_alias:
+            start_alias = self.query.get_initial_alias()
+        # The 'seen_models' is used to optimize checking the needed parent
+        # alias for a given field. This also includes None -> start_alias to
+        # be used by local fields.
+        seen_models = {None: start_alias}
+
         for field, model in opts.get_fields_with_model():
             if from_parent and model is not None and issubclass(from_parent, model):
                 # Avoid loading data for already loaded parents.
                 continue
-            alias = self.query.join_parent_model(opts, model, start_alias, seen)
+            alias = self.query.join_parent_model(opts, model, start_alias,
+                                                 seen_models)
             table = self.query.alias_map[alias].table_name
             if table in only_load and field.column not in only_load[table]:
                 continue

tests/regressiontests/select_related_regress/models.py

@@ -94,3 +94,17 @@ class Item(models.Model):
 
     def __str__(self):
         return self.name
+
+# Models for testing bug #19870.
+@python_2_unicode_compatible
+class Fowl(models.Model):
+    name = models.CharField(max_length=10)
+
+    def __str__(self):
+        return self.name
+
+class Hen(Fowl):
+    pass
+
+class Chick(Fowl):
+    mother = models.ForeignKey(Hen)

tests/regressiontests/select_related_regress/tests.py

@@ -5,7 +5,7 @@ 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)
+    Organizer, Class, Enrollment, Hen, Chick)
 
 
 class SelectRelatedRegressTests(TestCase):
@@ -162,3 +162,14 @@ class SelectRelatedRegressTests(TestCase):
             # The select_related join was promoted as there is already an
             # existing join.
             self.assertTrue('LEFT OUTER' in str(qs.query))
+
+    def test_regression_19870(self):
+        """
+        Regression for #19870
+
+        """
+        hen = Hen.objects.create(name='Hen')
+        chick = Chick.objects.create(name='Chick', mother=hen)
+
+        self.assertEqual(Chick.objects.all()[0].mother.name, 'Hen')
+        self.assertEqual(Chick.objects.select_related()[0].mother.name, 'Hen')