Change type in field

aggrate.js

var devices = db.getCollection('devices').aggregate([
    { "$group" : {_id:"$something" } }
]);

var batch = devices._batch;    
    
for(i=0; i<=batch.length; i++) {
    if (batch[i]) {
       if (batch[i]._id) {
           if (typeof(batch[i]._id) == 'number') {
               db.devices.update({something:batch[i]._id},{$set : { something: batch[i]._id.toString() } },{multi:true})
           }     
        }
    }
    
}