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

Thanks Simon Charette and Tim Graham for the reviews.

1 files changed, 15 insertions, 6 deletions

diff --git a/django/contrib/contenttypes/models.py b/django/contrib/contenttypes/models.py
index 0dc048cfa2..634576f4d0 100644
--- a/django/contrib/contenttypes/models.py
+++ b/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
-            # needed around opts.verbose_name_raw because name_raw might be a
-            # django.utils.functional.__proxy__ object.
+            pass
+
+        # 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):