try to add merge operation to aggregation pipeline

backend/routes/routesToucan.js

@@ -22,6 +22,7 @@ const urltoucans = Toucan.aggregate([
         {urlId:
             {$concat:["$titlelcnospace","-","$year"]}}}, 
     {$unset:["titlelc","year","titlelcnospace","title","date"]}, 
+    {$merge : "urls"}
 ]);
 
 router.route("/toucans")