Fix more vars, and re-worked Timestamp problems

backend/devices/service.py

@@ -1,7 +1,8 @@
 import secrets
 import string
 
-from google.cloud.firestore_v1 import GeoPoint
+from datetime import datetime
+from google.cloud.firestore_v1 import GeoPoint, DocumentReference
 
 from shared.firebase import get_db
 from shared.exceptions import NotFoundError
@@ -39,14 +40,15 @@ def _ensure_unique_serial(db) -> str:
 
 
 def _convert_firestore_value(val):
-    """Convert Firestore-specific types (Timestamp, GeoPoint) to strings."""
-    if hasattr(val, "seconds") and hasattr(val, "nanos"):
-        # Firestore Timestamp → ISO string
-        from datetime import datetime, timezone
-        dt = datetime.fromtimestamp(val.seconds + val.nanos / 1e9, tz=timezone.utc)
-        return dt.strftime("%d %B %Y at %H:%M:%S UTC")
+    """Convert Firestore-specific types (Timestamp, GeoPoint, DocumentReference) to strings."""
+    if isinstance(val, datetime):
+        # Firestore DatetimeWithNanoseconds is a datetime subclass
+        return val.strftime("%d %B %Y at %H:%M:%S UTC%z")
     if isinstance(val, GeoPoint):
         return f"{val.latitude}° N, {val.longitude}° E"
+    if isinstance(val, DocumentReference):
+        # Store the document path (e.g. "users/abc123")
+        return val.path
     return val