diff --git a/backend/app/tasks/scan.py b/backend/app/tasks/scan.py
index b7a8e91..78f67b4 100644
--- a/backend/app/tasks/scan.py
+++ b/backend/app/tasks/scan.py
@@ -618,43 +618,44 @@ async def handle_directory_rename(old_dirpath: str, new_dirpath: str) -> dict:
             )
             return {"status": "cross_root", "discarded": n}
 
-        # Same-root: prefix-rewrite. Use parameterised raw SQL so the
-        # SUBSTRING + concat happens server-side in one shot; iterating
-        # in Python would mean N row updates.
-        params = {
-            "new_prefix": new_prefix,
-            "old_prefix": old_prefix,
-            "off": len(old_prefix) + 1,
-            "old_pat": old_prefix + "/%",
-        }
-        photos_res = await session.execute(
-            text(
-                "UPDATE photos SET filepath = :new_prefix || SUBSTRING(filepath FROM :off) "
-                "WHERE filepath LIKE :old_pat"
-            ),
-            params,
-        )
-        folders_res = await session.execute(
-            text(
-                "UPDATE folders SET path = CASE "
-                "WHEN path = :old_prefix THEN :new_prefix "
-                "ELSE :new_prefix || SUBSTRING(path FROM :off) END "
-                "WHERE path = :old_prefix OR path LIKE :old_pat"
-            ),
-            params,
-        )
-        source_roots_res = await session.execute(
-            text(
-                "UPDATE source_roots SET path = :new_prefix WHERE path = :old_prefix"
-            ),
-            params,
-        )
+        # Same-root: iterate the matching rows in Python and rewrite
+        # the prefix attribute-side. We tried a single UPDATE … SET …
+        # SUBSTRING(... FROM LENGTH(:old)+1) raw-SQL approach but
+        # asyncpg miscategorises the LENGTH() result and rejects it
+        # as "$2: int (expected str)". The PATCH /folders/{id}
+        # endpoint already loops in Python for the same reason — match
+        # its pattern. Folder renames are rare and typically span ≤1k
+        # photos, so per-row UPDATEs are fine.
+        old_pat = old_prefix + "/%"
+        photos = (await session.execute(
+            select(Photo).where(Photo.filepath.like(old_pat))
+        )).scalars().all()
+        for p in photos:
+            p.filepath = new_prefix + p.filepath[len(old_prefix):]
+
+        folders = (await session.execute(
+            select(Folder).where(
+                or_(
+                    Folder.path == old_prefix,
+                    Folder.path.like(old_pat),
+                )
+            )
+        )).scalars().all()
+        for f in folders:
+            f.path = new_prefix if f.path == old_prefix else \
+                new_prefix + f.path[len(old_prefix):]
+
+        source_roots = (await session.execute(
+            select(SourceRoot).where(SourceRoot.path == old_prefix)
+        )).scalars().all()
+        for sr in source_roots:
+            sr.path = new_prefix
         await session.commit()
         return {
             "status": "renamed",
-            "photos": photos_res.rowcount or 0,
-            "folders": folders_res.rowcount or 0,
-            "source_roots": source_roots_res.rowcount or 0,
+            "photos": len(photos),
+            "folders": len(folders),
+            "source_roots": len(source_roots),
         }