feat: add API to return list of downstream blocks for an upstream [FC-0076] (#36253)

Adds the API for listing downstream contexts and parents for a given publishable entity.

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

@@ -40,6 +40,11 @@ https://github.com/openedx/edx-platform/issues/35653):
       400: Downstream block is not linked to upstream content.
       404: Downstream block not found or user lacks permission to edit it.
 
+  /api/contentstore/v2/upstream/{usage_key_string}/downstream-links
+
+    GET: List all downstream blocks linked to a library block.
+        200: A list of downstream usage_keys linked to the library block.
+
   # NOT YET IMPLEMENTED -- Will be needed for full Libraries Relaunch in ~Teak.
   /api/contentstore/v2/downstreams
   /api/contentstore/v2/downstreams?course_id=course-v1:A+B+C&ready_to_sync=true
@@ -71,7 +76,10 @@ from xblock.core import XBlock
 
 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,
@@ -138,6 +146,27 @@ class UpstreamListView(DeveloperErrorViewMixin, APIView):
         return Response(serializer.data)
 
 
+@view_auth_classes()
+class DownstreamContextListView(DeveloperErrorViewMixin, APIView):
+    """
+    Serves library block->downstream usage keys
+    """
+    def get(self, request: _AuthenticatedRequest, usage_key_string: str) -> Response:
+        """
+        Fetches downstream links for given publishable entity
+        """
+        try:
+            usage_key = UsageKey.from_string(usage_key_string)
+        except InvalidKeyError as exc:
+            raise ValidationError(detail=f"Malformed usage key: {usage_key_string}") from exc
+
+        links = PublishableEntityLink.get_by_upstream_usage_key(upstream_usage_key=usage_key)
+
+        serializer = PublishableEntityLinksUsageKeySerializer(links, many=True)
+
+        return Response(serializer.data)
+
+
 @view_auth_classes(is_authenticated=True)
 class DownstreamView(DeveloperErrorViewMixin, APIView):
     """

cms/djangoapps/contentstore/rest_api/v2/views/tests/test_downstreams.py

@@ -66,6 +66,20 @@ class _BaseDownstreamViewTestMixin:
         self.downstream_html_key = BlockFactory.create(
             category='html', parent=unit, upstream=MOCK_HTML_UPSTREAM_REF, upstream_version=1,
         ).usage_key
+
+        self.another_course = CourseFactory.create(display_name="Another Course")
+        another_chapter = BlockFactory.create(category="chapter", parent=self.another_course)
+        another_sequential = BlockFactory.create(category="sequential", parent=another_chapter)
+        another_unit = BlockFactory.create(category="vertical", parent=another_sequential)
+        self.another_video_keys = []
+        for _ in range(3):
+            # Adds 3 videos linked to the same upstream
+            self.another_video_keys.append(
+                BlockFactory.create(
+                    category="video", parent=another_unit, upstream=MOCK_UPSTREAM_REF, upstream_version=123,
+                ).usage_key
+            )
+
         self.fake_video_key = self.course.id.make_usage_key("video", "NoSuchVideo")
         self.superuser = UserFactory(username="superuser", password="password", is_staff=True, is_superuser=True)
         self.learner = UserFactory(username="learner", password="password")
@@ -341,3 +355,23 @@ class GetUpstreamViewTest(_BaseDownstreamViewTestMixin, SharedModuleStoreTestCas
             },
         ]
         self.assertListEqual(data, expected)
+
+
+class GetDownstreamContextsTest(_BaseDownstreamViewTestMixin, SharedModuleStoreTestCase):
+    """
+    Test that `GET /api/v2/contentstore/upstream/:usage_key/downstream-links returns list of
+    linked blocks usage_keys in given upstream entity (i.e. library block).
+    """
+    def call_api(self, usage_key_string):
+        return self.client.get(f"/api/contentstore/v2/upstream/{usage_key_string}/downstream-links")
+
+    def test_200_downstream_context_list(self):
+        """
+        Returns all downstream courses for given library block
+        """
+        self.client.login(username="superuser", password="password")
+        response = self.call_api(MOCK_UPSTREAM_REF)
+        assert response.status_code == 200
+        data = response.json()
+        expected = [str(self.downstream_video_key)] + [str(key) for key in self.another_video_keys]
+        self.assertListEqual(data, expected)