goosetest-604.js

var mongoose = require('./mongoose');
var Schema = mongoose.Schema;
mongoose.connect('localhost', 'testing_604');

// original git://gist.github.com/1352079.git

var CalendarEvent = new Schema({
   title      : String,
   allDay     : Boolean,
   start      : Date,
   end        : Date,
   url        : String,
   color      : String,
   ref : String
});

var CalendarMonth = new Schema({
 id          : Number,
 allEvents   : [CalendarEvent]
});

var CalendarEventReference = new Schema({
   id   : Number,  //Users id
   ref  : [CalendarMonth]
});

var A = mongoose.model('A', CalendarEventReference);
var CalendarEvent = mongoose.model('C', CalendarEvent);

mongoose.connection.on('open', function () {
  A.remove({}, function () {
    A.create({ id: 1, ref: [{id:9, allEvents:[]}] }, function (err) {
      if (err) return console.error(err.stack);

      var theEvent = new CalendarEvent();

      theEvent.title = 'my title'
      theEvent.allDay =  true
      theEvent.start = new Date
      theEvent.end =  new Date
      theEvent.url = "empty";
      theEvent.color = 'greeeeeen'
      theEvent.ref = 'the ref';

      A.findOne({ id: 1}, function(err, resultReference){
        if (err) return console.error(err.stack);

        resultReference.ref[0].allEvents.push(theEvent);
        resultReference.save(function(err){
          if (err) return console.error(err.stack);
          console.error('Succesfully saved');
          mongoose.connection.close();
        });
      });
    });
  });
});

I dont get undefined in the code I posted earlier

before i rearranged the order, the event was never saved properly

console.log(thing) // undefined
var thing = 'neato';