test.js

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

console.error(mongoose.version);

var ResponseSchema = new Schema({});

var AnswerSchema = new Schema({
    answer   : String
  , responses: [ResponseSchema]
});

function lengthValidator (v) {
  return v && !!v.length;
}

var QuestionSchema = new Schema({
    question: {type: String, validate: [lengthValidator, "can't be blank."]}
  , answers : [AnswerSchema]
});

var db = mongoose.createConnection('mongodb://localhost/question_'+ Date.now());

var Question = db.model('Question', QuestionSchema);

var q = new Question({ question: 'Change Me', answers: [{answer: 'Dr Pepper'}] });

q.save(function (err) {
  if (err) return done(err);
  console.error('saved question', q);

  var params = {
      question: 'TESTING'
    , answers:[
          { answer: 'Coke' }
        , { answer: 'Pepsi' }
        , { answer: '' }
      ]
  }

  var id = q.id;

  Question.update({_id : id}, params, function (err) {
    if (err) return done(err);
    Question.collection.findOne({ _id: new ObjectId(id) }, function (err, question) {
    //Question.findById(id, function (err, question) {
      if (err) return done(err);
      console.error('from db', question);
      done();
    });
  });
});


function done (err) {
  console.error('calling done');
  if (err) console.error(err);
  db.db.dropDatabase(function (err) {
    db.close();
    console.error('done');
  });
}