node_modules/mongoose/examples/population/population-of-existing-doc.js

var mongoose = require('../../lib');
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'
    },
    function(err, nintendo64) {
      if (err) return done(err);

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

/**
 * Population
 */

function example() {
  Game
  .findOne({name: /^Legend of Zelda/})
  .exec(function(err, ocinara) {
    if (err) return done(err);

    console.log('"%s" console _id: %s', ocinara.name, ocinara.consoles[0]);

    // population of existing document
    ocinara.populate('consoles', function(err) {
      if (err) return done(err);

      console.log(
        '"%s" was released for the %s on %s',
        ocinara.name,
        ocinara.consoles[0].name,
        ocinara.released.toLocaleDateString()
      );

      done();
    });
  });
}

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