Ensure _resolve_model does not return None

Author: beck

rest_framework/utils/model_meta.py

@@ -43,7 +43,11 @@ def _resolve_model(obj):
     """
     if isinstance(obj, six.string_types) and len(obj.split('.')) == 2:
         app_name, model_name = obj.split('.')
-        return models.get_model(app_name, model_name)
+        resolved_model = models.get_model(app_name, model_name)
+        if not resolved_model:
+            raise ValueError("Django did not return a model for "
+                             "{0}.{1}".format(app_name, model_name))
+        return resolved_model
     elif inspect.isclass(obj) and issubclass(obj, models.Model):
         return obj
     raise ValueError("{0} is not a Django model".format(obj))

tests/test_utils.py

@@ -7,6 +7,8 @@
 from rest_framework.views import APIView
 from tests.models import BasicModel
 
+import rest_framework.utils.model_meta
+
 
 class Root(APIView):
     pass
@@ -130,3 +132,34 @@ def test_resolve_non_django_model(self):
     def test_resolve_improper_string_representation(self):
         with self.assertRaises(ValueError):
             _resolve_model('BasicModel')
+
+
+class ResolveModelWithPatchedDjangoTests(TestCase):
+    """
+    Test coverage for when Django's `get_model` returns `None`.
+
+    Under certain circumstances Django may return `None` with `get_model`:
+    http://git.io/get-model-source
+
+    It usually happens with circular imports so it is important that DRF
+    excepts early, otherwise fault happens downstream and is much more
+    difficult to debug.
+
+    """
+
+    def setUp(self):
+        """Monkeypatch get_model."""
+        self.get_model = rest_framework.utils.model_meta.models.get_model
+
+        def get_model(app_label, model_name):
+            return None
+
+        rest_framework.utils.model_meta.models.get_model = get_model
+
+    def tearDown(self):
+        """Revert monkeypatching."""
+        rest_framework.utils.model_meta.models.get_model = self.get_model
+
+    def test_blows_up_if_model_does_not_resolve(self):
+        with self.assertRaises(ValueError):
+            _resolve_model('tests.BasicModel')