Mongoose return fields order inconsistent

I have a very simple mongoose query, that returns correctly but the field display ordering is not consistent. Notice how sometimes it lists _id and then username and sometimes it lists username and then _id. Any way to fix this?

exports.list = function(req, res){
    return db.userModel.find({}, 'username', function (err, users) {
         if(!err) {
             return res.json(users);
         } else {
            console.log(err);
            return res.send(400);
        }
    });
 }

Output

[
  {
    "username": "admin",
    "_id": "5145293a420135521c000001"
  },
  {
    "username": "bob",
    "_id": "5145293b420135521c000002"
  },
  {
    "_id": "5145293b420135521c000003",
    "username": "example"
  }
]