Optimize tag children retrieval

src/documents/serialisers.py

@@ -578,6 +578,10 @@ class TagSerializer(MatchingModelSerializer, OwnedObjectSerializer):
         ),
     )
     def get_children(self, obj):
+        children_map = self.context.get("children_map")
+        if children_map is not None:
+            children = children_map.get(obj.pk, [])
+        else:
             filter_q = self.context.get("document_count_filter")
             request = self.context.get("request")
             if filter_q is None:
@@ -585,7 +589,7 @@ class TagSerializer(MatchingModelSerializer, OwnedObjectSerializer):
                 filter_q = get_document_count_filter_for_user(user)
                 self.context["document_count_filter"] = filter_q
 
-        children_queryset = (
+            children = (
                 obj.get_children_queryset()
                 .select_related("owner")
                 .annotate(document_count=Count("documents", filter=filter_q))
@@ -593,15 +597,15 @@ class TagSerializer(MatchingModelSerializer, OwnedObjectSerializer):
 
             view = self.context.get("view")
             ordering = (
-            OrderingFilter().get_ordering(request, children_queryset, view)
+                OrderingFilter().get_ordering(request, children, view)
                 if request and view
                 else None
             )
             ordering = ordering or (Lower("name"),)
-        children_queryset = children_queryset.order_by(*ordering)
+            children = children.order_by(*ordering)
 
         serializer = TagSerializer(
-            children_queryset,
+            children,
             many=True,
             user=self.user,
             full_perms=self.full_perms,

src/documents/views.py

@@ -448,8 +448,43 @@ class TagViewSet(ModelViewSet, PermissionsAwareDocumentCountMixin):
     def get_serializer_context(self):
         context = super().get_serializer_context()
         context["document_count_filter"] = self.get_document_count_filter()
+        if hasattr(self, "_children_map"):
+            context["children_map"] = self._children_map
         return context
 
+    def list(self, request, *args, **kwargs):
+        """
+        Build a children map once to avoid per-parent queries in the serializer.
+        """
+        queryset = self.filter_queryset(self.get_queryset())
+        ordering = OrderingFilter().get_ordering(request, queryset, self) or (
+            Lower("name"),
+        )
+        queryset = queryset.order_by(*ordering)
+
+        all_tags = list(queryset)
+        descendant_pks = {pk for tag in all_tags for pk in tag.get_descendants_pks()}
+
+        if descendant_pks:
+            filter_q = self.get_document_count_filter()
+            children_source = (
+                Tag.objects.filter(pk__in=descendant_pks | {t.pk for t in all_tags})
+                .select_related("owner")
+                .annotate(document_count=Count("documents", filter=filter_q))
+                .order_by(*ordering)
+            )
+        else:
+            children_source = all_tags
+
+        children_map = {}
+        for tag in children_source:
+            children_map.setdefault(tag.tn_parent_id, []).append(tag)
+        self._children_map = children_map
+
+        page = self.paginate_queryset(queryset)
+        serializer = self.get_serializer(page, many=True)
+        return self.get_paginated_response(serializer.data)
+
     def perform_update(self, serializer):
         old_parent = self.get_object().get_parent()
         tag = serializer.save()