708.js

var mongoose = require('mongoose')
  , Schema = mongoose.Schema
  , tojson = mongoose.Document.prototype.toJSON;

mongoose.connect('localhost', 'testing_tojsonWithVirtuals');

var PersonSchema = new Schema({
    name    : String
  , age     : Number
});

PersonSchema.methods.toJSON = function (options) {
  var res = tojson.call(this, options);
  res.was_in_to_json = true;
  return res;
}

var StorySchema = new Schema({
    creators : [PersonSchema]
  , title    : String
});

StorySchema.methods.toJSON = PersonSchema.methods.toJSON;

var Story  = mongoose.model('Story', StorySchema);

mongoose.connection.on('open', function () {
  person1 = { name: 'Person1', age: 42 };
  person2 = { name: 'Person2', age: 42 };
  Story.create({ title: 'Test', creators: [person1, person2] }, function (err, story) {
    if (err) return console.error(err.stack||err);
    Story.findById(story).run(function (err, doc) {
      if (err) return console.error(err.stack||err);
      console.error(doc.toJSON());
      mongoose.connection.db.dropDatabase(function () {
        mongoose.connection.close();
      });
    });
  });
});