Model#update undefined

app.js

var express = require('express'),
	mongoose = require('mongoose'),
	Model = require('./lib/model'),
	Tap = Model.Tap;

app.post('/tap', function(req, res, next) {
	var tap = new Tap();
	tap.name = req.body.name;
	tap.beer = req.body.beer;
	tap.update({ name: tap.name }, { upsert: true }, function(err) {
		if (err) { next(err); return; }
		respUtil.sendJsonSuccess(res);
	});
});

model.js

var mongoose = require('mongoose'),
	Schema = mongoose.Schema,
	ObjectId = Schema.ObjectId;
	
// export our models
var Model = module.exports = {};

/**
 * Tab Schema
 */
var Tap = new Schema({
	name: { type: String, required: true },
	beer: { type: ObjectId, required: true },
	updated: { type: Date }
});
Tap.pre('save', function(next) {
	this.updated = Date.now();
	next();
});
Model.Tap = mongoose.model('Tap', Tap);