Get rid of n+1 queries

Author: mbertrand

learning_resources/models.py

@@ -386,10 +386,9 @@ def for_serialization(self, *, user: Optional["User"] = None):
                 ),
                 Prefetch(
                     "children",
-                    queryset=LearningResourceRelationship.objects.filter(
-                        relation_type=LearningResourceRelationTypes.PROGRAM_COURSES.value,
-                    ),
-                    to_attr="_courses",
+                    queryset=LearningResourceRelationship.objects.select_related(
+                        "child"
+                    ).order_by("position"),
                 ),
                 Prefetch(
                     "user_lists",
@@ -552,17 +551,6 @@ def podcasts(self) -> list["LearningResourceRelationship"]:
             ),
         )
 
-    @cached_property
-    def courses(self) -> list["LearningResourceRelationship"]:
-        """Return a list of courses that a resource contains"""
-        return getattr(
-            self,
-            "_courses",
-            self.children.filter(
-                relation_type=LearningResourceRelationTypes.PROGRAM_COURSES.value,
-            ).order_by("position"),
-        )
-
     class Meta:
         unique_together = (("platform", "readable_id", "resource_type"),)
 

learning_resources/serializers.py

@@ -444,26 +444,6 @@ class Meta:
         fields = ("id", "parent", "child")
 
 
-class MicroProgramRelationshipSerializer(serializers.ModelSerializer):
-    """
-    Serializer containing only parent and child ids for a program_course_relationship
-    """
-
-    child = serializers.ReadOnlyField(source="child_id")
-    readable_id = serializers.ReadOnlyField(source="child.readable_id")
-    title = serializers.ReadOnlyField(source="child.title")
-
-    class Meta:
-        model = models.LearningResourceRelationship
-        fields = (
-            "child",
-            "position",
-            "relation_type",
-            "title",
-            "readable_id",
-        )
-
-
 class LearningResourceMetadataDisplaySerializer(serializers.Serializer):
     """
     Serializer to render course information as a text document
@@ -930,13 +910,13 @@ class LearningResourceBaseSerializer(serializers.ModelSerializer, WriteableTopic
     resource_category = serializers.SerializerMethodField()
     format = serializers.ListField(child=FormatSerializer(), read_only=True)
     pace = serializers.ListField(child=PaceSerializer(), read_only=True)
-    children = MicroProgramRelationshipSerializer(many=True, read_only=True)
+    children = serializers.SerializerMethodField(allow_null=True)
 
-    # @extend_schema_field(LearningResourceRelationshipChildField(allow_null=True))
-    # def get_children(self, instance):
-    #     return LearningResourceRelationshipChildField(
-    #         instance.courses, many=True, read_only=True
-    #     ).data
+    @extend_schema_field(LearningResourceRelationshipChildField(allow_null=True))
+    def get_children(self, instance):
+        return LearningResourceRelationshipChildField(
+            instance.children, many=True, read_only=True
+        ).data
 
     def get_resource_category(self, instance) -> str:
         """Return the resource category of the resource"""

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