fix: fix the issue of nested field access

backend/models/base_doc.py

@@ -15,6 +15,10 @@ class QueryExpression:
     def __init__(self, field_name: str):
         self.field_name = field_name
 
+    def __getattr__(self, name: str) -> 'QueryExpression':
+        """Handle nested field access like FLID.identity"""
+        return QueryExpression(f"{self.field_name}.{name}")
+
     def __eq__(self, other: Any) -> Dict[str, Any]:
         """Handle field == value comparisons"""
         return {self.field_name: other}