TNL-3316: Error on Insights performance report

Preserve DRF v2 behavior of defaulting to None
for missing keys in the grading policy dictionary.

lms/djangoapps/course_structure_api/v0/tests.py

@@ -36,6 +36,23 @@ class CourseViewTestsMixin(object):
     """
     view = None
 
+    raw_grader = [
+        {
+            "min_count": 24,
+            "weight": 0.2,
+            "type": "Homework",
+            "drop_count": 0,
+            "short_label": "HW"
+        },
+        {
+            "min_count": 4,
+            "weight": 0.8,
+            "type": "Exam",
+            "drop_count": 0,
+            "short_label": "Exam"
+        }
+    ]
+
     def setUp(self):
         super(CourseViewTestsMixin, self).setUp()
         self.create_user_and_access_token()
@@ -51,22 +68,7 @@ class CourseViewTestsMixin(object):
     @classmethod
     def create_course_data(cls):
         cls.invalid_course_id = 'foo/bar/baz'
-        cls.course = CourseFactory.create(display_name='An Introduction to API Testing', raw_grader=[
-            {
-                "min_count": 24,
-                "weight": 0.2,
-                "type": "Homework",
-                "drop_count": 0,
-                "short_label": "HW"
-            },
-            {
-                "min_count": 4,
-                "weight": 0.8,
-                "type": "Exam",
-                "drop_count": 0,
-                "short_label": "Exam"
-            }
-        ])
+        cls.course = CourseFactory.create(display_name='An Introduction to API Testing', raw_grader=cls.raw_grader)
         cls.course_id = unicode(cls.course.id)
         with cls.store.bulk_operations(cls.course.id, emit_signals=False):
             cls.sequential = ItemFactory.create(
@@ -408,6 +410,55 @@ class CourseGradingPolicyTests(CourseDetailTestMixin, CourseViewTestsMixin, Shar
         self.assertListEqual(response.data, expected)
 
 
+class CourseGradingPolicyMissingFieldsTests(CourseDetailTestMixin, CourseViewTestsMixin, SharedModuleStoreTestCase):
+    view = 'course_structure_api:v0:grading_policy'
+
+    # Update the raw grader to have missing keys
+    raw_grader = [
+        {
+            "min_count": 24,
+            "weight": 0.2,
+            "type": "Homework",
+            "drop_count": 0,
+            "short_label": "HW"
+        },
+        {
+            # Deleted "min_count" key
+            "weight": 0.8,
+            "type": "Exam",
+            "drop_count": 0,
+            "short_label": "Exam"
+        }
+    ]
+
+    @classmethod
+    def setUpClass(cls):
+        super(CourseGradingPolicyMissingFieldsTests, cls).setUpClass()
+        cls.create_course_data()
+
+    def test_get(self):
+        """
+        The view should return grading policy for a course.
+        """
+        response = super(CourseGradingPolicyMissingFieldsTests, self).test_get()
+
+        expected = [
+            {
+                "count": 24,
+                "weight": 0.2,
+                "assignment_type": "Homework",
+                "dropped": 0
+            },
+            {
+                "count": None,
+                "weight": 0.8,
+                "assignment_type": "Exam",
+                "dropped": 0
+            }
+        ]
+        self.assertListEqual(response.data, expected)
+
+
 #####################################################################################
 #
 # The following Mixins/Classes collectively test the CourseBlocksAndNavigation view.

openedx/core/djangoapps/content/course_structures/api/v0/serializers.py

 """
 API Serializers
 """
+from collections import defaultdict
+
 from rest_framework import serializers
 
 
@@ -11,6 +13,19 @@ class GradingPolicySerializer(serializers.Serializer):
     dropped = serializers.IntegerField(source='drop_count')
     weight = serializers.FloatField()
 
+    def to_representation(self, obj):
+        """
+        Return a representation of the grading policy.
+        """
+        # Backwards compatibility with the behavior of DRF v2.
+        # When the grader dictionary was missing keys, DRF v2 would default to None;
+        # DRF v3 unhelpfully raises an exception.
+        return dict(
+            super(GradingPolicySerializer, self).to_representation(
+                defaultdict(lambda: None, obj)
+            )
+        )
+
 
 # pylint: disable=invalid-name
 class BlockSerializer(serializers.Serializer):