window.Site = Backbone.RelationalModel.extend({
    idAttribute: "_id",
    // These are the relations to the user model.
    relations: [{
        type: Backbone.HasMany,
        key: 'users',
        relatedModel: 'window.User'
      }],

});

//Site User model
  // -------------

  window.User = Backbone.RelationalModel.extend({
    });

{
_id: foo,
users: [
username: bar,
password: fizz
]}

initialize: function() {

  //basic bindings
  this.model.bind('change', this.setSave, this);
  this.model.bind('destroy', this.remove, this);

  // bindings to sub-models
  this.model.bind('add:users', this.setDetailsView, this);
  this.model.bind('remove:users', this.setSave, this);
  this.model.bind('change:users', this.setSave, this);

initialize: function() {
  this.model.bind('change', this.render, this);
  this.model.bind('destroy', this.remove, this);

this.model.get('users').each(function(user){
    user.bind(change,this.render,this);
},this);