fix: return the downstream_usage_key list using a serializer

cms/djangoapps/contentstore/rest_api/v2/serializers/__init__.py

@@ -1,4 +1,7 @@
 """Module for v2 serializers."""
 
-from cms.djangoapps.contentstore.rest_api.v2.serializers.downstreams import PublishableEntityLinksSerializer
+from cms.djangoapps.contentstore.rest_api.v2.serializers.downstreams import (
+    PublishableEntityLinksSerializer,
+    PublishableEntityLinksUsageKeySerializer,
+)
 from cms.djangoapps.contentstore.rest_api.v2.serializers.home import CourseHomeTabSerializerV2

cms/djangoapps/contentstore/rest_api/v2/serializers/downstreams.py

@@ -26,3 +26,15 @@ def get_ready_to_sync(self, obj):
     class Meta:
         model = PublishableEntityLink
         exclude = ['upstream_block', 'uuid']
+
+
+class PublishableEntityLinksUsageKeySerializer(serializers.ModelSerializer):
+    """
+    Serializer for returning a string list of the usage keys.
+    """
+    def to_representation(self, instance: PublishableEntityLink) -> str:
+        return str(instance.downstream_usage_key)
+
+    class Meta:
+        model = PublishableEntityLink
+        fields = ('downstream_usage_key')

cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py

@@ -76,7 +76,10 @@
 
 from cms.djangoapps.contentstore.helpers import import_static_assets_for_library_sync
 from cms.djangoapps.contentstore.models import PublishableEntityLink
-from cms.djangoapps.contentstore.rest_api.v2.serializers import PublishableEntityLinksSerializer
+from cms.djangoapps.contentstore.rest_api.v2.serializers import (
+    PublishableEntityLinksSerializer,
+    PublishableEntityLinksUsageKeySerializer,
+)
 from cms.lib.xblock.upstream_sync import (
     BadDownstream,
     BadUpstream,
@@ -156,15 +159,14 @@ def get(self, request: _AuthenticatedRequest, usage_key_string: str) -> Response
         except InvalidKeyError as exc:
             raise ValidationError(detail=f"Malformed usage key: {usage_key_string}") from exc
 
-        downstream_usage_key_list = (
+        links = (
             PublishableEntityLink
             .get_by_upstream_usage_key(upstream_usage_key=usage_key)
-            .values_list("downstream_usage_key", flat=True)
         )
 
-        downstream_usage_key_str_list = [str(usage_key) for usage_key in downstream_usage_key_list]
+        serializer = PublishableEntityLinksUsageKeySerializer(links, many=True)
 
-        return Response(downstream_usage_key_str_list)
+        return Response(serializer.data)
 
 
 @view_auth_classes(is_authenticated=True)