gistfile1.txt

/* sites.js */
var 
	mongoose = require('mongoose')
	, Schema = mongoose.Schema
	, update = require('./updates')
;

//Define the site model
var siteSchema = new Schema({
	client: String,						
	siteUrl: String,
	siteMaintenanceRequirements: String,
	siteLastMaintenance: [update],
	siteContractActive: Boolean
});
	
module.exports = mongoose.model('site', siteSchema);


/* updates.js */
var 
	mongoose = require('mongoose')
	, Schema = mongoose.Schema
	, ObjectId = Schema.ObjectId
;

//Define the site model
var updateSchema = new Schema({
	maintenanceDescription: String,
	person: String,
	date: Date
});
	
module.exports = mongoose.model('update', updateSchema);

The idea is to have a simple one-to-many relation, there should be multiple updates per site.