mongodb aggregation project objectId with concat

Question

db.test.aggregate({
    $match : { "themType" : "SuperTest" , "mType" : { "$in" : [ 1 , 2]}}
}, 
{ $project : { "_id" : 1, "refTestId" : 1, "avatar" : { $concat : [$refTestId] }
  } });

and avatar returns me null, probably its because its objectId, is it possible in this query to make from this objectId string ?

Answer 1

This isn't possible yet. WiP issue see: https://jira.mongodb.org/browse/SERVER-29512