/**
  * When a new order is inserted, add the subscriptions period to the order. 
  * 
  * It's important that the period is tied to the order so that changing the products
  * period does not change the past. 
  *
  * @since 1.0
  */
 public static function add_order_meta($order_id)
 {
     global $woocommerce;
     if (WC_Subscriptions_Order::order_contains_subscription($order_id)) {
         $order = new WC_Order($order_id);
         $order_subscription_periods = array();
         $order_subscription_intervals = array();
         $order_subscription_lengths = array();
         $order_subscription_trial_lengths = array();
         foreach ($order->get_items() as $item) {
             $period = WC_Subscriptions_Product::get_period($item['id']);
             if (!empty($period)) {
                 $order_subscription_periods[$item['id']] = $period;
             }
             $interval = WC_Subscriptions_Product::get_interval($item['id']);
             if (!empty($interval)) {
                 $order_subscription_intervals[$item['id']] = $interval;
             }
             $length = WC_Subscriptions_Product::get_length($item['id']);
             if (!empty($length)) {
                 $order_subscription_lengths[$item['id']] = $length;
             }
             $trial_length = WC_Subscriptions_Product::get_trial_length($item['id']);
             if (!empty($trial_length)) {
                 $order_subscription_trial_lengths[$item['id']] = $trial_length;
             }
         }
         update_post_meta($order_id, '_order_subscription_periods', $order_subscription_periods);
         update_post_meta($order_id, '_order_subscription_intervals', $order_subscription_intervals);
         update_post_meta($order_id, '_order_subscription_lengths', $order_subscription_lengths);
         update_post_meta($order_id, '_order_subscription_trial_lengths', $order_subscription_trial_lengths);
         // Store sign-up fee details
         foreach (WC_Subscriptions_Cart::get_sign_up_fee_fields() as $field_name) {
             update_post_meta($order_id, "_{$field_name}", $woocommerce->cart->{$field_name});
         }
         // Prepare sign up fee taxes to store in same format as order taxes
         $sign_up_fee_taxes = array();
         foreach (array_keys($woocommerce->cart->sign_up_fee_taxes) as $key) {
             $is_compound = $woocommerce->cart->tax->is_compound($key) ? 1 : 0;
             $sign_up_fee_taxes[] = array('label' => $woocommerce->cart->tax->get_rate_label($key), 'compound' => $is_compound, 'cart_tax' => number_format($woocommerce->cart->sign_up_fee_taxes[$key], 2, '.', ''));
         }
         update_post_meta($order_id, '_sign_up_fee_taxes', $sign_up_fee_taxes);
     }
 }