thoughts/node_modules/mongoose/examples/population-of-multiple-existing-docs.js

var mongoose = require('mongoose')
var Schema = mongoose.Schema;

console.log('Running mongoose version %s', mongoose.version);

/**
 * Console schema
 */

var consoleSchema = Schema({
    name: String
  , manufacturer: String
  , released: Date
})
var Console = mongoose.model('Console', consoleSchema);

/**
 * Game schema
 */

var gameSchema = Schema({
    name: String
  , developer: String
  , released: Date
  , consoles: [{ type: Schema.Types.ObjectId, ref: 'Console' }]
})
var Game = mongoose.model('Game', gameSchema);

/**
 * Connect to the console database on localhost with
 * the default port (27017)
 */

mongoose.connect('mongodb://localhost/console', function (err) {
  // if we failed to connect, abort
  if (err) throw err;

  // we connected ok
  createData();
})

/**
 * Data generation
 */

function createData () {
  Console.create({
      name: 'Nintendo 64'
    , manufacturer: 'Nintendo'
    , released: 'September 29, 1996'
  }, {
      name: 'Super Nintendo'
    , manufacturer: 'Nintendo'
    , released: 'August 23, 1991'
  }, function (err, nintendo64, superNintendo) {
    if (err) return done(err);

    Game.create({
        name: 'Legend of Zelda: Ocarina of Time'
      , developer: 'Nintendo'
      , released: new Date('November 21, 1998')
      , consoles: [nintendo64]
    }, {
        name: 'Mario Kart'
      , developer: 'Nintendo'
      , released: 'September 1, 1992'
      , consoles: [superNintendo]
    }, function (err) {
      if (err) return done(err);
      example();
    })
  })
}

/**
 * Population
 */

function example () {
  Game
  .find({})
  .exec(function (err, games) {
    if (err) return done(err);

    console.log('found %d games', games.length);

    var options = { path: 'consoles', select: 'name released -_id' };
    Game.populate(games, options, function (err, games) {
      if (err) return done(err);

      games.forEach(function (game) {
        console.log(
            '"%s" was released for the %s on %s'
          , game.name
          , game.consoles[0].name
          , game.released.toLocaleDateString());
      })

      done()
    })
  })
}

function done (err) {
  if (err) console.error(err);
  Console.remove(function () {
    Game.remove(function () {
      mongoose.disconnect();
    })
  })
}