Add enrollment_course_run to entitlementserializer

The marketing site currently cannot use the entitlement
api to determine whether a user has an unfilled entitlement.
Including the entitlement enrollment_course_run makes this
information accessible to consumers of the entitlement API.

Learner-3731

common/djangoapps/entitlements/api/v1/serializers.py

@@ -6,6 +6,10 @@ from entitlements.models import CourseEntitlement
 
 class CourseEntitlementSerializer(serializers.ModelSerializer):
     user = serializers.SlugRelatedField(slug_field='username', queryset=get_user_model().objects.all())
+    enrollment_course_run = serializers.CharField(
+        source='enrollment_course_run.course_id',
+        read_only=True
+    )
 
     class Meta:
         model = CourseEntitlement
@@ -13,6 +17,7 @@ class CourseEntitlementSerializer(serializers.ModelSerializer):
             'user',
             'uuid',
             'course_uuid',
+            'enrollment_course_run',
             'expired_at',
             'created',
             'modified',

common/djangoapps/entitlements/api/v1/tests/test_serializers.py

@@ -27,6 +27,7 @@ class EntitlementsSerializerTests(ModuleStoreTestCase):
             'expired_at': entitlement.expired_at,
             'course_uuid': str(entitlement.course_uuid),
             'mode': entitlement.mode,
+            'enrollment_course_run': None,
             'order_number': entitlement.order_number,
             'created': entitlement.created.strftime('%Y-%m-%dT%H:%M:%S.%fZ'),
             'modified': entitlement.modified.strftime('%Y-%m-%dT%H:%M:%S.%fZ'),

common/djangoapps/entitlements/api/v1/tests/test_views.py

@@ -159,8 +159,8 @@ class EntitlementViewSetTest(ModuleStoreTestCase):
     @patch("entitlements.api.v1.views.get_course_runs_for_course")
     def test_add_entitlement_inactive_audit_enrollment(self, mock_get_course_runs):
         """
-        Verify that if an entitlement is added for a user, if the user has one upgradeable enrollment
-        that enrollment is upgraded to the mode of the entitlement and linked to the entitlement.
+        Verify that if an entitlement is added for a user, if the user has an inactive audit enrollment
+        that enrollment is NOT upgraded to the mode of the entitlement and linked to the entitlement.
         """
         course_uuid = uuid.uuid4()
         entitlement_data = self._get_data_set(self.user, str(course_uuid))