diff --git a/opensensor/collection_apis.py b/opensensor/collection_apis.py
index beceb7f..00bc9b0 100644
--- a/opensensor/collection_apis.py
+++ b/opensensor/collection_apis.py
@@ -1,6 +1,6 @@
 import logging
 from datetime import datetime, timedelta, timezone
-from typing import Generic, List, Optional, Type, TypeVar, get_args, get_origin
+from typing import Dict, Generic, List, Optional, Type, TypeVar, get_args, get_origin
 
 from bson import Binary
 from fastapi import APIRouter, Depends, HTTPException, Path, Query, Response, status
@@ -187,7 +187,7 @@ def is_pydantic_model(obj):
     return isinstance(obj, type) and issubclass(obj, BaseModel)
 
 
-def get_nested_fields(model: Type[BaseModel]):
+def get_nested_fields(model: Type[BaseModel]) -> Dict[str, Type[BaseModel]]:
     nested_fields = {}
     for field_name, field in model.__fields__.items():
         if is_pydantic_model(field.type_):
@@ -206,7 +206,7 @@ def create_nested_pipeline(model: Type[BaseModel], prefix=""):
         "timestamp": "$timestamp",
     }
 
-    for field_name, field_type in model.__fields__.items():
+    for field_name, field in model.__fields__.items():
         if field_name == "timestamp":
             continue
         lookup_field = (
@@ -219,31 +219,30 @@ def create_nested_pipeline(model: Type[BaseModel], prefix=""):
             unit_field_name = f"{prefix}{mongo_field}_unit"
             pipeline["unit"] = f"${unit_field_name}"
             match_conditions[unit_field_name] = {"$exists": True}
-        else:
-            pipeline[field_name] = f"${full_mongo_field_name}"
-            match_conditions[full_mongo_field_name] = {"$exists": True}
-
-        if field_name in nested_fields:
-            if get_origin(field_type.type_) is List:
-                nested_pipeline, nested_match = create_nested_pipeline(
-                    nested_fields[field_name], ""  # Empty prefix for list items
-                )
+        elif field_name in nested_fields:
+            nested_type = nested_fields[field_name]
+            if get_origin(field.type_) is List:
+                # Handle array of nested objects
+                nested_pipeline, nested_match = create_nested_pipeline(nested_type, "")
                 pipeline[field_name] = {
                     "$map": {
                         "input": f"${full_mongo_field_name}",
                         "as": "item",
-                        "in": {
-                            k: f"$$item.{v.replace('$', '')}" for k, v in nested_pipeline.items()
-                        },
+                        "in": nested_pipeline,
                     }
                 }
                 match_conditions[full_mongo_field_name] = {"$exists": True, "$ne": []}
             else:
+                # Handle nested object
                 nested_pipeline, nested_match = create_nested_pipeline(
-                    nested_fields[field_name], f"{field_name}."
+                    nested_type, f"{field_name}."
                 )
                 pipeline[field_name] = nested_pipeline
                 match_conditions.update({f"{field_name}.{k}": v for k, v in nested_match.items()})
+        else:
+            # Handle simple field
+            pipeline[field_name] = f"${full_mongo_field_name}"
+            match_conditions[full_mongo_field_name] = {"$exists": True}
 
         logger.debug(f"Field: {field_name}, Full mongo field name: {full_mongo_field_name}")
         logger.debug(f"Resulting pipeline part: {pipeline[field_name]}")