Reduce the number of queries used in tagging API [FC-00036] (#157)

* fix: copy Django's internal prefetch cache when casting taxonomies to preserve any prefetch_related data that's available.
* fix: prevents TaxonomyTagsView from running get_queryset twice
  Overrides the permissions._queryset method to avoid using view.get_queryset(), since that method accesses the database.
  Also renames the permission class to better represent the view it guards.
* fix: annotate taxonomies with their tags_count instead of prefetching all the tags.
* chore: bumps version

openedx_learning/__init__.py

@@ -1,4 +1,4 @@
 """
 Open edX Learning ("Learning Core").
 """
-__version__ = "0.6.1"
+__version__ = "0.6.2"

openedx_tagging/core/tagging/models/base.py

@@ -372,6 +372,12 @@ def copy(self, taxonomy: Taxonomy) -> Taxonomy:
         self.visible_to_authors = taxonomy.visible_to_authors
         self.export_id = taxonomy.export_id
         self._taxonomy_class = taxonomy._taxonomy_class  # pylint: disable=protected-access
+
+        # Copy Django's internal prefetch_related cache to reduce queries required on the casted taxonomy.
+        if hasattr(taxonomy, '_prefetched_objects_cache'):
+            # pylint: disable=protected-access,attribute-defined-outside-init
+            self._prefetched_objects_cache: dict = taxonomy._prefetched_objects_cache
+
         return self
 
     def get_filtered_tags(

openedx_tagging/core/tagging/rest_api/v1/permissions.py

@@ -4,7 +4,7 @@
 import rules  # type: ignore[import]
 from rest_framework.permissions import DjangoObjectPermissions
 
-from ...models import Tag
+from ...models import Tag, Taxonomy
 
 
 class TaxonomyObjectPermissions(DjangoObjectPermissions):
@@ -37,9 +37,9 @@ class ObjectTagObjectPermissions(DjangoObjectPermissions):
     }
 
 
-class TagObjectPermissions(DjangoObjectPermissions):
+class TaxonomyTagsObjectPermissions(DjangoObjectPermissions):
     """
-    Maps each REST API methods to its corresponding Tag permission.
+    Maps each REST API methods to its corresponding Taxonomy permission.
     """
     perms_map = {
         "GET": ["%(app_label)s.view_%(model_name)s"],
@@ -58,3 +58,11 @@ def has_object_permission(self, request, view, obj):
         """
         obj = obj.taxonomy if isinstance(obj, Tag) else obj
         return rules.has_perm("oel_tagging.list_tag", request.user, obj)
+
+    def _queryset(self, view):
+        """
+        Returns the queryset to use when checking model and object permissions.
+
+        The base class method calls view.get_queryset(), but that method performs database queries, so we override it.
+        """
+        return Taxonomy.objects

openedx_tagging/core/tagging/rest_api/v1/serializers.py

@@ -100,6 +100,13 @@ def to_representation(self, instance):
         return super().to_representation(instance)
 
     def get_tags_count(self, instance):
+        """
+        Return the "tags_count" annotation if present.
+
+        Or just count the taxonomy's tags.
+        """
+        if hasattr(instance, 'tags_count'):
+            return instance.tags_count
         return instance.tag_set.count()
 
     def get_can_tag_object(self, instance) -> bool | None:

openedx_tagging/core/tagging/rest_api/v1/views.py

@@ -33,7 +33,7 @@
 from ...rules import ObjectTagPermissionItem
 from ..paginators import MAX_FULL_DEPTH_THRESHOLD, DisabledTagsPagination, TagsPagination, TaxonomyPagination
 from ..utils import view_auth_classes
-from .permissions import ObjectTagObjectPermissions, TagObjectPermissions, TaxonomyObjectPermissions
+from .permissions import ObjectTagObjectPermissions, TaxonomyObjectPermissions, TaxonomyTagsObjectPermissions
 from .serializers import (
     ObjectTagListQueryParamsSerializer,
     ObjectTagsByTaxonomySerializer,
@@ -252,7 +252,9 @@ def get_queryset(self) -> models.QuerySet:
         query_params.is_valid(raise_exception=True)
         enabled = query_params.data.get("enabled", None)
 
-        return get_taxonomies(enabled)
+        qs = get_taxonomies(enabled)
+        qs = qs.annotate(tags_count=models.Count("tag", distinct=True))
+        return qs
 
     def perform_create(self, serializer) -> None:
         """
@@ -724,7 +726,7 @@ class TaxonomyTagsView(ListAPIView, RetrieveUpdateDestroyAPIView):
 
     """
 
-    permission_classes = [TagObjectPermissions]
+    permission_classes = [TaxonomyTagsObjectPermissions]
     pagination_class = TagsPagination
     serializer_class = TagDataSerializer
 

tests/openedx_tagging/core/tagging/test_views.py

@@ -257,7 +257,7 @@ def test_list_taxonomy_query_count(self):
         url = TAXONOMY_LIST_URL
 
         self.client.force_authenticate(user=self.user)
-        with self.assertNumQueries(5):
+        with self.assertNumQueries(3):
             response = self.client.get(url)
 
         assert response.status_code == 200
@@ -1461,7 +1461,7 @@ def test_small_query_count(self):
         url = f"{self.small_taxonomy_url}?search_term=eU"
 
         self.client.force_authenticate(user=self.staff)
-        with self.assertNumQueries(5):
+        with self.assertNumQueries(4):
             response = self.client.get(url)
 
         assert response.status_code == status.HTTP_200_OK