mongoose virtual attributes

mongoose_virtual_attr.js

// http://mongoosejs.com/docs/virtuals.html
// npm install mongoose underscore

var _ = require('underscore');
var mongoose = require('mongoose');

mongoose.connect(process.env.MONGOLAB_URI || process.env.MONGOHQ_URL || 'mongodb://localhost/test', function(err){
  if(err){
    console.error(err);
    process.exit(1);
  }
});

var Schema = mongoose.Schema, ObjectId = Schema.ObjectId;

var RecipeSchema = new Schema({
  id : {type: String, unique: true},
  title : {type: String},
  created_at : {type: Date, default: Date.now}
});
RecipeSchema.virtual('url').get(function(){
  return 'http://shokai.org/'+this.id;
});

var Recipe = mongoose.model('Recipe', RecipeSchema);
Recipe.latests = function(num){
  return this.find().sort('created_at', -1).limit(num);
};

var recipe = new Recipe({id: '29jg', title: '肉じゃが'});
recipe.save();

Recipe.latests(3).exec(function(err, docs){
  if(err){
    throw err;
  }
  else{
    _.each(docs, function(doc){
      console.log(doc.url);
    });
  }
  mongoose.disconnect();
});

result.txt

% node mongoose_virtual_attr.js
http://shokai.org/29jg

how about the performence?