mattdrayer/api: Fixed NRE corner case for grades leaderboard

lms/djangoapps/api_manager/courses/tests.py

@@ -187,6 +187,11 @@ class CoursesApiTests(TestCase):
             display_name=u"test unit 2",
         )
 
+        self.empty_course = CourseFactory.create(
+            start=datetime(2014, 6, 16, 14, 30),
+            end=datetime(2015, 1, 16),
+            org="MTD"
+        )
 
         self.users = [UserFactory.create(username="testuser" + str(__), profile='test') for __ in xrange(USER_COUNT)]
 
@@ -1630,6 +1635,13 @@ class CoursesApiTests(TestCase):
         response = self.do_get(bogus_test_uri)
         self.assertEqual(response.status_code, 404)
 
+    def test_courses_metrics_grades_leaders_list_get_empty_course(self):
+        test_uri = '{}/{}/metrics/grades/leaders/'.format(self.base_courses_uri, unicode(self.empty_course.id))
+        response = self.do_get(test_uri)
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.data['course_avg'], 0)
+        self.assertEqual(len(response.data['leaders']), 0)
+
     def test_courses_completions_leaders_list_get(self):
         completion_uri = '{}/{}/completions/'.format(self.base_courses_uri, unicode(self.course.id))
         # Make last user as observer to make sure that data is being filtered out
@@ -1722,6 +1734,14 @@ class CoursesApiTests(TestCase):
         self.assertEqual(response.status_code, 200)
         self.assertEqual(len(response.data['grades']), user_index)
 
+    def test_courses_metrics_grades_list_get_empty_course(self):
+        # Retrieve the list of grades for this course
+        # All the course/item/user scaffolding was handled in Setup
+        test_uri = '{}/{}/metrics/grades'.format(self.base_courses_uri, unicode(self.empty_course.id))
+        response = self.do_get(test_uri)
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response.data['grade_count'], 0)
+        self.assertEqual(response.data['course_grade_maximum'], 0)
 
     def test_courses_grades_list_get_invalid_course(self):
         # Retrieve the list of grades for this course

lms/djangoapps/gradebook/models.py

@@ -56,11 +56,21 @@ class StudentGradebook(TimeStampedModel):
         have not yet submitted a response to a scored assessment which means no grade has been calculated.
         """
         data = {}
+        data['course_avg'] = 0
+        data['course_max'] = 0
+        data['course_min'] = 0
+        data['course_count'] = 0
+        data['queryset'] = []
+
         total_user_count = CourseEnrollment.users_enrolled_in(course_key).count()
+        if total_user_count:
+
+            # Generate the base data set we're going to work with
             queryset = StudentGradebook.objects.select_related('user')\
                 .filter(course_id__exact=course_key, user__is_active=True)
             gradebook_user_count = len(queryset)
 
+            # Remove any users who should not be considered for the statistics
             if exclude_users:
                 total_user_count = total_user_count - len(exclude_users)
                 queryset = queryset.exclude(user__in=exclude_users)
@@ -71,6 +81,8 @@ class StudentGradebook(TimeStampedModel):
             if gradebook_user_count < total_user_count:
                 # Take into account any ungraded students (assumes zeros for grades...)
                 course_avg = course_avg / total_user_count * gradebook_user_count
+
+            # Fill up the response container
             data['course_avg'] = course_avg
             data['course_max'] = queryset.aggregate(Max('grade'))['grade__max']
             data['course_min'] = queryset.aggregate(Min('grade'))['grade__min']