String concatenation mongodb in array [требует правки]

I'm trying to convert array elements to new lines in the document, but I always get null values That's what I have now.

{
"_id" : ObjectId("5af9a044db06595c4a3c60d3"),
"word" : [ 
    "1501143005", 
    "41D44F7F-D5B3-4f3f-85AC-1E122DA8C506", 
    "4100", 
    "7493"
]}

Here's what I'm trying to get

{
"_id" : ObjectId("5af9a044db06595c4a3c60d3"),
"newId" : "1501143005-4100",
"oldId" : "7493-4100"}      

So far I have come to the next construction

db.getCollection('buffer2').aggregate([
{ 
   $project:{
      word:{
          "$map":{
              "input": "$word", "as" : "u",
              "in":{
        newId: { $concat: [ "$$u.0", "-", "$$u.2" ] },
        oldId: { $concat: [ "$$u.3", "-", "$$u.2" ] } 
       }
       }
   }}
   }]);

I would be grateful for help