Fixed #20401 -- ContentTypeManager.get_for_model reads from db_for_read.

Thanks Simon Charette and Tim Graham for the reviews.

django/contrib/contenttypes/models.py

@@ -38,18 +38,27 @@ class ContentTypeManager(models.Manager):
         """
         opts = self._get_opts(model, for_concrete_model)
         try:
-            ct = self._get_from_cache(opts)
+            return self._get_from_cache(opts)
         except KeyError:
-            # Load or create the ContentType entry. The smart_text() is
+            pass
-            # needed around opts.verbose_name_raw because name_raw might be a
+
-            # django.utils.functional.__proxy__ object.
+        # The ContentType entry was not found in the cache, therefore we
+        # proceed to load or create it.
+        try:
+            # We start with get() and not get_or_create() in order to use
+            # the db_for_read (see #20401).
+            ct = self.get(app_label=opts.app_label, model=opts.model_name)
+        except self.model.DoesNotExist:
+            # Not found in the database; we proceed to create it.  This time we
+            # use get_or_create to take care of any race conditions.
+            # The smart_text() is needed around opts.verbose_name_raw because
+            # name_raw might be a django.utils.functional.__proxy__ object.
             ct, created = self.get_or_create(
                 app_label=opts.app_label,
                 model=opts.model_name,
                 defaults={'name': smart_text(opts.verbose_name_raw)},
             )
-            self._add_to_cache(self.db, ct)
+        self._add_to_cache(self.db, ct)
-
         return ct
 
     def get_for_models(self, *models, **kwargs):

tests/contenttypes_tests/tests.py

@@ -7,7 +7,7 @@ from django.contrib.contenttypes.fields import (
 )
 from django.contrib.contenttypes.models import ContentType
 from django.core import checks
-from django.db import models
+from django.db import connections, models, router
 from django.test import TestCase
 from django.test.utils import override_settings
 from django.utils.encoding import force_str
@@ -335,3 +335,40 @@ class GenericRelationshipTests(IsolatedModelsTestCase):
             )
         ]
         self.assertEqual(errors, expected)
+
+
+class TestRouter(object):
+    def db_for_read(self, model, **hints):
+        return 'other'
+
+    def db_for_write(self, model, **hints):
+        return 'default'
+
+
+class ContentTypesMultidbTestCase(TestCase):
+
+    def setUp(self):
+        self.old_routers = router.routers
+        router.routers = [TestRouter()]
+
+        # Whenever a test starts executing, only the "default" database is
+        # connected. We explicitly connect to the "other" database here. If we
+        # don't do it, then it will be implicitly connected later when we query
+        # it, but in that case some database backends may automatically perform
+        # extra queries upon connecting (notably mysql executes
+        # "SET SQL_AUTO_IS_NULL = 0"), which will affect assertNumQueries().
+        connections['other'].ensure_connection()
+
+    def tearDown(self):
+        router.routers = self.old_routers
+
+    def test_multidb(self):
+        """
+        Test that, when using multiple databases, we use the db_for_read (see
+        #20401).
+        """
+        ContentType.objects.clear_cache()
+
+        with self.assertNumQueries(0, using='default'), \
+                self.assertNumQueries(1, using='other'):
+            ContentType.objects.get_for_model(Author)