Return null for empty learner fields.

analytics_data_api/v0/serializers.py

@@ -332,6 +332,14 @@ class LearnerSerializer(serializers.Serializer, DefaultIfNoneMixin):
     enrollment_date = serializers.DateField(source='enrollment_date', format=settings.DATE_FORMAT)
     cohort = serializers.CharField(source='cohort')
 
+    def transform_segments(self, _obj, value):
+        # returns null instead of empty strings
+        return value or []
+
+    def transform_cohort(self, _obj, value):
+        # returns null instead of empty strings
+        return value or None
+
     def get_account_url(self, obj):
         if settings.LMS_USER_ACCOUNT_BASE_URL:
             return urljoin(settings.LMS_USER_ACCOUNT_BASE_URL, obj.username)

analytics_data_api/v0/tests/views/test_learners.py

@@ -118,25 +118,32 @@ class LearnerAPITestMixin(object):
             enrollment_date='2015-01-28',
     ):
         """Create a single learner roster entry in the elasticsearch index."""
-        self._es.create(
-            index=settings.ELASTICSEARCH_LEARNERS_INDEX,
-            doc_type='roster_entry',
-            body={
+        body = {
             'username': username,
             'course_id': course_id,
             'name': name if name is not None else username,
             'email': email if email is not None else '{}@example.com'.format(username),
             'enrollment_mode': enrollment_mode,
-                'segments': segments if segments is not None else list(),
-                'cohort': cohort,
             'discussion_contributions': discussion_contributions,
             'problems_attempted': problems_attempted,
             'problems_completed': problems_completed,
-                'problem_attempts_per_completed': problem_attempts_per_completed,
             'attempt_ratio_order': attempt_ratio_order,
             'videos_viewed': videos_viewed,
             'enrollment_date': enrollment_date,
         }
+
+        # leave null fields from being stored in the index.  Otherwise, they will have
+        # an explicit null value and we want to test for the case when they're not returned
+        optional_fields = [('segments', segments), ('cohort', cohort),
+                           ('problem_attempts_per_completed', problem_attempts_per_completed)]
+        for optional_field in optional_fields:
+            if optional_field[1]:
+                body[optional_field[0]] = optional_field[1]
+
+        self._es.create(
+            index=settings.ELASTICSEARCH_LEARNERS_INDEX,
+            doc_type='roster_entry',
+            body=body
         )
 
     def create_learners(self, learners):