Stopped calling apps.get_model with only_installed=False.

ContentTypes are only created for installed applications, and I could
make a case for not returning a model that isn't installed any more.
The check for stale ContentTypes in update_contenttypes doesn't use
model_class.

ModelSignal actually needs get_registered_model since the lookup happens
at import time. I took this opportunity to perform a small refactoring.

django/contrib/contenttypes/models.py

@@ -157,7 +157,7 @@ class ContentType(models.Model):
 
     def model_class(self):
         "Returns the Python model class for this type of content."
-        return apps.get_model(self.app_label, self.model, only_installed=False)
+        return apps.get_model(self.app_label, self.model)
 
     def get_object_for_this_type(self, **kwargs):
         """

django/db/models/signals.py

@@ -1,5 +1,3 @@
-from collections import defaultdict
-
 from django.apps import apps
 from django.dispatch import Signal
 from django.utils import six
@@ -16,7 +14,7 @@ class ModelSignal(Signal):
 
     def __init__(self, *args, **kwargs):
         super(ModelSignal, self).__init__(*args, **kwargs)
-        self.unresolved_references = defaultdict(list)
+        self.unresolved_references = {}
         class_prepared.connect(self._resolve_references)
 
     def _resolve_references(self, sender, **kwargs):
@@ -35,18 +33,17 @@ class ModelSignal(Signal):
     def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):
         if isinstance(sender, six.string_types):
             try:
-                app_label, object_name = sender.split('.')
+                app_label, model_name = sender.split('.')
             except ValueError:
                 raise ValueError(
                     "Specified sender must either be a model or a "
                     "model name of the 'app_label.ModelName' form."
                 )
-            sender = apps.get_model(app_label, object_name, only_installed=False)
+            sender = apps.get_registered_model(app_label, model_name)
             if sender is None:
-                reference = (app_label, object_name)
-                self.unresolved_references[reference].append(
-                    (receiver, weak, dispatch_uid)
-                )
+                ref = (app_label, model_name)
+                refs = self.unresolved_references.setdefault(ref, [])
+                refs.append((receiver, weak, dispatch_uid))
                 return
         super(ModelSignal, self).connect(
             receiver, sender=sender, weak=weak, dispatch_uid=dispatch_uid

tests/app_loading/tests.py

@@ -75,12 +75,6 @@ class GetModelsTest(TestCase):
         self.assertEqual(
             apps.get_model("not_installed", "NotInstalledModel"), None)
 
-    def test_get_model_with_not_installed(self):
-        self.assertEqual(
-            apps.get_model(
-                "not_installed", "NotInstalledModel", only_installed=False),
-            self.not_installed_module.NotInstalledModel)
-
     def test_get_models_only_returns_installed_models(self):
         self.assertNotIn(
             "NotInstalledModel",