refactor: promote sort_field_map to class-level constant on TantivyBackend

Single source of truth for sort field mapping. The viewset now references
TantivyBackend.SORTABLE_FIELDS instead of maintaining a duplicate set.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/documents/search/_backend.py

@@ -234,6 +234,34 @@ class TantivyBackend:
     the underlying index directory changes (e.g., during test isolation).
     """
 
+    # Maps DRF ordering field names to Tantivy index field names.
+    SORT_FIELD_MAP: dict[str, str] = {
+        "title": "title_sort",
+        "correspondent__name": "correspondent_sort",
+        "document_type__name": "type_sort",
+        "created": "created",
+        "added": "added",
+        "modified": "modified",
+        "archive_serial_number": "asn",
+        "page_count": "page_count",
+        "num_notes": "num_notes",
+    }
+
+    # Fields where Tantivy's sort order matches the ORM's sort order.
+    # Text-based fields (title, correspondent__name, document_type__name)
+    # are excluded because Tantivy's tokenized fast fields produce different
+    # ordering than the ORM's collation-based ordering.
+    SORTABLE_FIELDS: frozenset[str] = frozenset(
+        {
+            "created",
+            "added",
+            "modified",
+            "archive_serial_number",
+            "page_count",
+            "num_notes",
+        },
+    )
+
     def __init__(self, path: Path | None = None):
         # path=None → in-memory index (for tests)
         # path=some_dir → on-disk index (for production)
@@ -487,22 +515,9 @@ class TantivyBackend:
         searcher = self._index.searcher()
         offset = (page - 1) * page_size
 
-        # Map sort fields
-        sort_field_map = {
-            "title": "title_sort",
-            "correspondent__name": "correspondent_sort",
-            "document_type__name": "type_sort",
-            "created": "created",
-            "added": "added",
-            "modified": "modified",
-            "archive_serial_number": "asn",
-            "page_count": "page_count",
-            "num_notes": "num_notes",
-        }
-
         # Perform search
-        if sort_field and sort_field in sort_field_map:
-            mapped_field = sort_field_map[sort_field]
+        if sort_field and sort_field in self.SORT_FIELD_MAP:
+            mapped_field = self.SORT_FIELD_MAP[sort_field]
             results = searcher.search(
                 final_query,
                 limit=offset + page_size,

src/documents/views.py

@@ -2059,6 +2059,7 @@ class UnifiedSearchViewSet(DocumentViewSet):
             return super().list(request)
 
         from documents.search import SearchMode
+        from documents.search import TantivyBackend
         from documents.search import TantivyRelevanceList
         from documents.search import get_backend
 
@@ -2083,20 +2084,9 @@ class UnifiedSearchViewSet(DocumentViewSet):
             sort_reverse = ordering_param.startswith("-")
             sort_field_name = ordering_param.lstrip("-") if ordering_param else None
 
-            # Fields Tantivy can sort natively — only numeric/date fast fields.
-            # Text-based sorts (title, correspondent__name, document_type__name)
-            # use a tokenized fast field whose ordering may differ from the ORM,
-            # so they fall back to the ORM sort path.
-            tantivy_sortable = {
-                "created",
-                "added",
-                "modified",
-                "archive_serial_number",
-                "page_count",
-                "num_notes",
-            }
             use_tantivy_sort = (
-                sort_field_name in tantivy_sortable or sort_field_name is None
+                sort_field_name in TantivyBackend.SORTABLE_FIELDS
+                or sort_field_name is None
             )
 
             # Compute the DRF page so we can tell Tantivy which slice to highlight