Features: 1) Add Prefetch import in filters.py; 2) Introduce filter_order_by method with custom ordering logic for Category model.

Fixes: 1) Replace `OrderingFilter` with `CharFilter` for `order_by` field in `CategoryFilter`.

Extra: 1) Remove unused ordering configuration from `order_by` field; 2) Refactor ordering logic with prefetch-related optimization for children categories.

core/filters.py

@@ -15,6 +15,7 @@ from django.db.models import (
     Value,
     When,
     Max,
+    Prefetch,
 )
 from django.db.models.functions import Coalesce
 from django.utils.http import urlsafe_base64_decode
@@ -382,14 +383,7 @@ class CategoryFilter(FilterSet):
     tags = CaseInsensitiveListFilter(field_name="tags__tag_name", label=_("Tags"))
     level = NumberFilter(field_name="level", lookup_expr="exact", label=_("Level"))
 
-    order_by = OrderingFilter(
+    order_by = CharFilter(method="filter_order_by")
-        fields=(
-            ("priority", "priority"),
-            ("uuid", "uuid"),
-            ("name", "name"),
-            ("?", "random"),
-        )
-    )
 
     class Meta:
         model = Category
@@ -404,6 +398,35 @@ class CategoryFilter(FilterSet):
             "whole",
         ]
 
+    def filter_order_by(self, queryset, _name, value):
+        if not value:
+            return queryset
+
+        desc = value.startswith("-")
+        key = value.lstrip("-")
+
+        mapping = {
+            "priority": "priority",
+            "uuid": "uuid",
+            "name": "name",
+            "?": "random",
+        }
+        field = mapping.get(key)
+        if field is None:
+            return queryset
+
+        if field == "?":
+            parent_order = "?"
+            child_order = "?"
+        else:
+            parent_order = f"-{field}" if desc else field
+            child_order = parent_order
+
+        qs = queryset.order_by(parent_order)
+        children_qs = Category.objects.all().order_by(child_order)
+
+        return qs.prefetch_related(Prefetch("children", queryset=children_qs))
+
     def filter_name(self, queryset, _name, value):
         search_results = process_query(query=value, request=self.request)["categories"]
         uuids = [res["uuid"] for res in search_results if res.get("uuid")]