http://stuffpetedoes.blogspot.nl/2012/07/nodeunit-mongoose-and-grunt-watch.html

fixedGreetee.test.js

var mongoose = require('./testUtils/mongooseTestWrapper.js'),
	Greetee;

exports.setUp = function(callback) {
	// reset the schemas to ensure that any changes are picked up by the mongoose singleton
	mongoose.resetSchemas();
	
	// add the schemas back again
	Greetee = require('./greetee.js');

	// connect to a test database and drop all the greetees from it
	mongoose.connect('mongodb://localhost/UnitTest_Greetee');
	Greetee.remove({}, function(err) {
		callback();
	});
};

...

greetee.js

var mongoose = require('mongoose'),
	Schema = mongoose.Schema;

var GreeteeSchema = new Schema({
	name: String
});

GreeteeSchema.methods.greeting = function() {
	return 'Hello, ' + this.name;
};

module.exports = mongoose.model('Greetee', GreeteeSchema);

greetee.test.js

var mongoose = require('mongoose'),
	Greetee = require('./greetee.js');

exports.setUp = function(callback) {
	// connect to a test database and drop all the greetees from it
	mongoose.connect('mongodb://localhost/UnitTest_Greetee');
	Greetee.remove({}, function(err) {
		callback();
	});
};

exports.tearDown = function(callback) {
	mongoose.disconnect();
	callback();
};

exports.greeting = function(test) {
	test.expect(1);
	var greetee = new Greetee({
		name: 'Pete'
	});
	test.equal('Hello, Pete', greetee.greeting());
	test.done();
};

mongooseTestWrapper.js

var mongoose = require('mongoose');
mongoose.resetSchemas = function() {
	this.modelSchemas = {};
	this.models = {};
};
module.exports = mongoose;

newGreetee.js

...

GreeteeSchema.methods.greeting = function() {
	return 'Bonjour, ' + this.name;
};

...

newGreetee.test.js

...

exports.greeting = function(test) {
	test.expect(1);
	var greetee = new Greetee({
		name: 'Pete'
	});
	test.equal('Bonjour, Pete', greetee.greeting());
	test.done();
};