Avoid n+1 queries on video.playlists serializer field (#2662)

Author: mbertrand

learning_resources/filters_test.py

@@ -446,15 +446,18 @@ def test_learning_resource_filter_course_features(client):
     """Test that the resource_content_tag filter works"""
 
     resource_with_exams = LearningResourceFactory.create(
-        content_tags=LearningResourceContentTagFactory.create_batch(1, name="Exams")
+        content_tags=LearningResourceContentTagFactory.create_batch(1, name="Exams"),
+        resource_type=LearningResourceType.video.name,
     )
     resource_with_notes = LearningResourceFactory.create(
         content_tags=LearningResourceContentTagFactory.create_batch(
             1, name="Lecture Notes"
-        )
+        ),
+        resource_type=LearningResourceType.video.name,
     )
     LearningResourceFactory.create(
-        content_tags=LearningResourceContentTagFactory.create_batch(1, name="Other")
+        content_tags=LearningResourceContentTagFactory.create_batch(1, name="Other"),
+        resource_type=LearningResourceType.video.name,
     )
 
     results = client.get(f"{RESOURCE_API_URL}?course_feature=exams").json()["results"]

learning_resources/models.py

@@ -390,6 +390,13 @@ def for_serialization(self, *, user: Optional["User"] = None):
                 ),
                 to_attr="_podcasts",
             ),
+            Prefetch(
+                "parents",
+                queryset=LearningResourceRelationship.objects.filter(
+                    relation_type=LearningResourceRelationTypes.PLAYLIST_VIDEOS.value,
+                ),
+                to_attr="_playlists",
+            ),
             Prefetch(
                 "children",
                 queryset=LearningResourceRelationship.objects.select_related(
@@ -604,6 +611,17 @@ def podcasts(self) -> list["LearningResourceRelationship"]:
             ),
         )
 
+    @cached_property
+    def playlists(self) -> list["LearningResourceRelationship"]:
+        """Return a list of playlists that the resource is in"""
+        return getattr(
+            self,
+            "_playlists",
+            self.parents.filter(
+                relation_type=LearningResourceRelationTypes.PLAYLIST_VIDEOS.value,
+            ),
+        )
+
     class Meta:
         unique_together = (("platform", "readable_id", "resource_type"),)
 

learning_resources/serializers.py

@@ -1097,11 +1097,7 @@ class VideoResourceSerializer(LearningResourceBaseSerializer):
 
     def get_playlists(self, instance) -> list[str]:
         """Get the playlist id(s) the video belongs to"""
-        return list(
-            instance.parents.filter(
-                relation_type=constants.LearningResourceRelationTypes.PLAYLIST_VIDEOS.value
-            ).values_list("parent__id", flat=True)
-        )
+        return [playlist.parent_id for playlist in instance.playlists]
 
 
 class VideoPlaylistResourceSerializer(LearningResourceBaseSerializer):

learning_resources/serializers_test.py

@@ -33,7 +33,11 @@
     LearningResourcePriceFactory,
     LearningResourceRunFactory,
 )
-from learning_resources.models import ContentFile, LearningResource
+from learning_resources.models import (
+    ContentFile,
+    LearningResource,
+    LearningResourceRelationship,
+)
 from main.test_utils import assert_json_equal, drf_datetime
 from main.utils import frontend_absolute_url
 
@@ -141,6 +145,36 @@ def test_serialize_podcast_episode_to_json():
     )
 
 
+def test_serialize_video_resource_playlists_to_json():
+    """
+    Verify that a serialized video resource has the correct playlist data
+    """
+    playlist = factories.VideoPlaylistFactory.create()
+    video = factories.VideoFactory.create()
+    LearningResourceRelationship.objects.get_or_create(
+        parent=playlist.learning_resource,
+        child=video.learning_resource,
+        relation_type=LearningResourceRelationTypes.PLAYLIST_VIDEOS.value,
+    )
+    serializer = serializers.VideoResourceSerializer(instance=video.learning_resource)
+    assert serializer.data["playlists"] == [playlist.learning_resource.id]
+
+
+def test_serialize_podcast_episode_playlists_to_json():
+    """
+    Verify that a serialized podcast episode resource has the correct podcast data
+    """
+    podcast = factories.PodcastFactory.create()
+    podcast_episode = factories.PodcastEpisodeFactory.create()
+    LearningResourceRelationship.objects.get_or_create(
+        parent=podcast.learning_resource,
+        child=podcast_episode.learning_resource,
+        relation_type=LearningResourceRelationTypes.PODCAST_EPISODES.value,
+    )
+    serializer = serializers.PodcastEpisodeSerializer(instance=podcast_episode)
+    assert serializer.data["podcasts"] == [podcast.learning_resource.id]
+
+
 @pytest.mark.parametrize("has_context", [True, False])
 @pytest.mark.parametrize(
     ("params", "detail_key", "specific_serializer_cls", "detail_serializer_cls"),

learning_resources/views_test.py

@@ -194,7 +194,7 @@ def test_program_detail_endpoint(client, django_assert_num_queries, url):
     """Test program endpoint"""
     program = ProgramFactory.create()
     assert program.learning_resource.children.count() > 0
-    with django_assert_num_queries(20):  # should be same # regardless of child count
+    with django_assert_num_queries(21):  # should be same # regardless of child count
         resp = client.get(reverse(url, args=[program.learning_resource.id]))
     assert resp.data.get("title") == program.learning_resource.title
     assert resp.data.get("resource_type") == LearningResourceType.program.name
@@ -239,7 +239,7 @@ def test_no_excess_queries(rf, user, mocker, django_assert_num_queries, course_c
     request = rf.get("/")
     request.user = user
 
-    with django_assert_num_queries(22):
+    with django_assert_num_queries(23):
         view = CourseViewSet(request=request)
         results = view.get_queryset().all()
         assert len(results) == course_count