Get rid of n+1 queries in learning_resources API

learning_resources/models.py

@@ -384,6 +384,12 @@ def for_serialization(self, *, user: Optional["User"] = None):
                     ),
                     to_attr="_podcasts",
                 ),
+                Prefetch(
+                    "children",
+                    queryset=LearningResourceRelationship.objects.select_related(
+                        "child"
+                    ).order_by("position"),
+                ),
                 Prefetch(
                     "user_lists",
                     queryset=UserListRelationship.objects.filter(parent__author=user)

learning_resources/serializers.py

@@ -915,7 +915,7 @@ class LearningResourceBaseSerializer(serializers.ModelSerializer, WriteableTopic
     @extend_schema_field(LearningResourceRelationshipChildField(allow_null=True))
     def get_children(self, instance):
         return LearningResourceRelationshipChildField(
-            instance.children.order_by("position"), many=True, read_only=True
+            instance.children, many=True, read_only=True
         ).data
 
     def get_resource_category(self, instance) -> str:

learning_resources/views_test.py

@@ -174,7 +174,8 @@ def test_program_endpoint(client, url, params):
 def test_program_detail_endpoint(client, django_assert_num_queries, url):
     """Test program endpoint"""
     program = ProgramFactory.create()
-    with django_assert_num_queries(18 + program.learning_resource.children.count()):
+    assert program.learning_resource.children.count() > 0
+    with django_assert_num_queries(18):  # 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
@@ -219,7 +220,7 @@ 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(19):
+    with django_assert_num_queries(20):
         view = CourseViewSet(request=request)
         results = view.get_queryset().all()
         assert len(results) == course_count