Assets belong to a model

app.js

// asset.js updated to have parentId and assetClass
var AssetSchema = new Schema({
  urls: [AssetURLSchema],
  md5: { type: String },
  name: { type: String },
  mimetype: { type: String },
  family: { type: String },
  format: { type: String },
  uploaded: { type: Date, 'default': Date.now },
  assetClass: { type: String, enum: ['image', 'font'] },
  parentId: { type: Schema.Types.ObjectId }
});


// imageAsset.js has a convenience method for finding the parent publication
ImageAsset.prototype.getPublication = function(done) {
  return Publication.findById(this.asset.parentId, done);
};

// And a simple example of working with it

// create an asset
var asset = new Asset({
  assetClass: 'image',
  parentId: publication.id
});

asset.save()

// Later, retrieve it and create an ImageAsset instance

Asset.findById(myId, function(err, asset) {
  var image = new ImageAsset(asset);

  image.getPublication(function(err, publication) {
     // do something with a publication here
  })
})