fix: check None before returning config (#296)

* fix: check None before returning config

* squash! add testcase

CHANGELOG.rst

@@ -16,6 +16,7 @@ Unreleased
 
 * Switch from ``edx-sphinx-theme`` to ``sphinx-book-theme`` since the former is
   deprecated
+* Fixed ConfigurationModel.current: it will make sure that it does not return None for current configuration.
 
 [2.3.0] - 2022-01-19
 ~~~~~~~~~~~~~~~~~~~~

config_models/models.py

@@ -131,7 +131,7 @@ def current(cls, *args):
         """
         cache_key = cls.cache_key_name(*args)
         cached_response = TieredCache.get_cached_response(cache_key)
-        if cached_response.is_found:
+        if cached_response.is_found and cached_response.value is not None:
             return cached_response.value
 
         key_dict = dict(zip(cls.KEY_FIELDS, args))

tests/test_config_models.py

@@ -1,10 +1,11 @@
 """
 Tests of ConfigurationModel
 """
-
+from unittest import mock
 
 import ddt
 from django.contrib.auth import get_user_model
+from edx_django_utils.cache.utils import CachedResponse
 from example.models import (ExampleConfig, ExampleKeyedConfig,
                             ManyToManyExampleConfig)
 from freezegun import freeze_time
@@ -45,6 +46,21 @@ def test_no_config_empty_cache(self):
             self.assertEqual(cached_current.int_field, 10)
             self.assertEqual(cached_current.string_field, '')
 
+    @mock.patch('config_models.models.TieredCache.get_cached_response')
+    def test_config_with_cached_response_value_none(self, mock_cached_response):
+        mock_cached_response.return_value = CachedResponse(is_found=True, key='test-key', value=None)
+        # First time reads from the database
+        with self.assertNumQueries(1):
+            current = ExampleConfig.current()
+            self.assertEqual(current.int_field, 10)
+            self.assertEqual(current.string_field, '')
+
+        # Follow-on request reads from database instead of cache as cache value will be None.
+        with self.assertNumQueries(1):
+            cached_current = ExampleConfig.current()
+            self.assertEqual(cached_current.int_field, 10)
+            self.assertEqual(cached_current.string_field, '')
+
     def test_config_ordering(self):
         with freeze_time('2012-01-01'):
             first = ExampleConfig(changed_by=self.user)