Mongoose: toJSON() on detached entities

model.js

var FeedSchema = new Schema({
   title: String,
   description: String,
   link: String,
   rss: String,
   pubdate: Date,
   updated: { type: Date, default: Date.now },
   items: [{type: Schema.Types.ObjectId, ref: 'Item'}],
});

var ItemSchema = new Schema({
   title: String,
   description: String,
   link: String,
   source: String,
   guid: String,
   author: String,
   pubdate: Date,
   updated: { type: Date, default: Date.now },
   _feed: { type: Schema.Types.ObjectId, ref: 'Feed' }
});

mongoose.model('Feed', FeedSchema);
mongoose.model('Item', ItemSchema);

-------------

var Feed = mongoose.model('Feed');
var feed = new Feed();
feed.items.push(new Item());

console.log(feed.toJSON());

output

{
  "title": "BBC News - Home",
  "description": "The latest stories from the Home section of the BBC News web site.",
  "pubdate": "2013-10-18T01:30:10.000Z",
  "link": "http://www.bbc.co.uk/news/#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa",
  "_id": "52609041036001b379000052",
  "items": [
    "52609041036001b379000053"
  ],
  "updated": "2013-10-18T01:34:57.167Z",
  "created": "2013-10-18T01:34:57.000Z",
  "id": "52609041036001b379000052"
}