Mongoose.js：填充模型实例的附加字段

Question

说我有一个架构

var userSchema = new mongoose.Schema({
    email: 'string',
    password: 'string,
    age: {type: 'number', select: false}

})

User.find({...}).exec().then(function(user){

  //user.age -> is undefined
  ...
  // LATER I want "age" to be filled from DB

  //So I may do (I may also wrap in custom model method)
  User.findById(user._id, 'age').exec().then(function(userOnlyAge){
     user.age = userOnlyAge.age
     ..
  })

})

最好的方法是什么？我想知道它是否可以开箱即用？