gradebook appears to bomb if there is no data to aggregate

lms/djangoapps/gradebook/models.py

@@ -77,42 +77,44 @@ class StudentGradebook(TimeStampedModel):
                     queryset = queryset.exclude(user__in=exclude_users)
                     gradebook_user_count = len(queryset)
 
-                # Calculate the class average
-                course_avg = queryset.aggregate(Avg('grade'))['grade__avg']
-                if course_avg is not None and 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'] = float("{0:.3f}".format(course_avg))
-                data['course_max'] = queryset.aggregate(Max('grade'))['grade__max']
-                data['course_min'] = queryset.aggregate(Min('grade'))['grade__min']
-                data['course_count'] = queryset.aggregate(Count('grade'))['grade__count']
-                # Construct the leaderboard as a queryset
-                data['queryset'] = queryset.values(
-                    'user__id',
-                    'user__username',
-                    'user__profile__title',
-                    'user__profile__avatar_url',
-                    'grade',
-                    'modified')\
-                    .order_by('-grade', 'modified')[:count]
-                # If a user_id value was provided, we need to provide some additional user-specific data to the caller
-                if user_id:
-                    user_grade = 0
-                    user_time_scored = timezone.now()
-                    try:
-                        user_queryset = StudentGradebook.objects.get(course_id__exact=course_key, user__id=user_id)
-                    except StudentGradebook.DoesNotExist:
-                        user_queryset = None
-                    if user_queryset:
-                        user_grade = user_queryset.grade
-                        user_time_scored = user_queryset.created
-                    users_above = queryset.filter(grade__gte=user_grade)\
-                        .exclude(user__id=user_id)\
-                        .exclude(grade=user_grade, modified__gt=user_time_scored)
-                    data['user_position'] = len(users_above) + 1
-                    data['user_grade'] = user_grade
+                if gradebook_user_count:
+                    # Calculate the class average
+                    course_avg = queryset.aggregate(Avg('grade'))['grade__avg']
+                    if course_avg is not None and 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'] = float("{0:.3f}".format(course_avg))
+                        data['course_max'] = queryset.aggregate(Max('grade'))['grade__max']
+                        data['course_min'] = queryset.aggregate(Min('grade'))['grade__min']
+                        data['course_count'] = queryset.aggregate(Count('grade'))['grade__count']
+
+                    # Construct the leaderboard as a queryset
+                    data['queryset'] = queryset.values(
+                        'user__id',
+                        'user__username',
+                        'user__profile__title',
+                        'user__profile__avatar_url',
+                        'grade',
+                        'modified')\
+                        .order_by('-grade', 'modified')[:count]
+                    # If a user_id value was provided, we need to provide some additional user-specific data to the caller
+                    if user_id:
+                        user_grade = 0
+                        user_time_scored = timezone.now()
+                        try:
+                            user_queryset = StudentGradebook.objects.get(course_id__exact=course_key, user__id=user_id)
+                        except StudentGradebook.DoesNotExist:
+                            user_queryset = None
+                        if user_queryset:
+                            user_grade = user_queryset.grade
+                            user_time_scored = user_queryset.created
+                        users_above = queryset.filter(grade__gte=user_grade)\
+                            .exclude(user__id=user_id)\
+                            .exclude(grade=user_grade, modified__gt=user_time_scored)
+                        data['user_position'] = len(users_above) + 1
+                        data['user_grade'] = user_grade
 
         return data