$resource relations in Angular.js [updated]

I'm trying to figure out the best way to write a model that relates to other models, such as an Order that has 1 or more OrderItems.

How can I get respective OrderItems when a Order is loaded?

    .module('MyApp.services', ['ngResource'])
        .factory('Order', function($resource) {
            return $resource('/api/v1/Order/:orderId?format=json', {}, {});
        .factory('OrderItem', function($resource) {
            return $resource('/api/v1/OrderItem/:orderitemId?format=json', {}, {});

I tried a callback function on get Order to load OrderItems, but didn't work.

there is a very similar question, but it is obsolete: $resource relations in Angular.js

You could wrap the function in another that gets order items?

myApp.factory('Order', function($resource) {
  var res = $resource('/api/Order/:orderId', {}, {
    '_get': { method: 'GET' }

  res.get = function(params, success, error) {
    return res._get(params, function(data) { 
    }, error);
  return res;

Before Andy's answer, I was workarounding it in Controllers. To do this, just add:

function OrderCtrl($scope, $routeParams, $resource, Order, OrderItem) {

    $scope.order = Order.get({
        orderId : $routeParams.orderId
    }, function(order) {