Fix created_at probelm for MongoDB vector storage

lightrag/kg/mongo_impl.py

@@ -999,9 +999,14 @@ class MongoVectorDBStorage(BaseVectorStorage):
         if not data:
             return
 
+        # Add current time as Unix timestamp
+        import time
+        current_time = int(time.time())
+        
         list_data = [
             {
                 "_id": k,
+                "created_at": current_time,  # Add created_at field as Unix timestamp
                 **{k1: v1 for k1, v1 in v.items() if k1 in self.meta_fields},
             }
             for k, v in data.items()
@@ -1059,9 +1064,14 @@ class MongoVectorDBStorage(BaseVectorStorage):
         cursor = self._data.aggregate(pipeline)
         results = await cursor.to_list()
 
-        # Format and return the results
+        # Format and return the results with created_at field
         return [
-            {**doc, "id": doc["_id"], "distance": doc.get("score", None)}
+            {
+                **doc, 
+                "id": doc["_id"], 
+                "distance": doc.get("score", None),
+                "created_at": doc.get("created_at")  # Include created_at field
+            }
             for doc in results
         ]
 
@@ -1152,8 +1162,15 @@ class MongoVectorDBStorage(BaseVectorStorage):
             cursor = self._data.find({"_id": {"$regex": f"^{prefix}"}})
             matching_records = await cursor.to_list(length=None)
 
-            # Format results
-            results = [{**doc, "id": doc["_id"]} for doc in matching_records]
+            # Format results, ensuring created_at is included
+            results = [
+                {
+                    **doc, 
+                    "id": doc["_id"],
+                    "created_at": doc.get("created_at")  # Include created_at field
+                } 
+                for doc in matching_records
+            ]
 
             logger.debug(
                 f"Found {len(results)} records with prefix '{prefix}' in {self.namespace}"