<?php
/**
 * Implements hook_menu().
 *
 */
function rebuild_ajax_form_menu() {
  $items['rebuild_ajax_form'] = array(
    'page callback' => 'rebuild_ajax_form_rebuild_form',
    'page arguments' => array(1),
    'type' => MENU_CALLBACK,
    'access callback' => TRUE,
    'theme callback' => 'ajax_base_page_theme',
  );
	return $items;
}

function rebuild_ajax_form_rebuild_form($product_id) {
  if (isset($product_id) && isset($_POST['form_build_id'])) {
    // Get form from cache if it exists
    $form_state = form_state_defaults();
    $form = form_get_cache($_POST['form_build_id'], $form_state);
    if (!$form) {
      // Form not found, rebuild form and update form_id via AJAX
      // This implementation will depend entirely on your use case.
      // In our case, the form is rendered by means of 'field_custom',
      // so we reload the form by loading and rendering the field
      $node = node_load((int) $product_id);
      $output = field_view_field('node', $node, 'field_custom');
      $form = $output[0];
      // Keep track of the old `form_build_id`
      $form['#build_id_old'] = $_POST['form_build_id'];
      // Replace `form_build_id` with updated one
      $commands[] = ajax_command_update_build_id($form);
      $return = array(
        '#type' => 'ajax',
        '#commands' => $commands,
      );
      ajax_deliver($return);
    }
  }
}

/**
 * Implements hook_form_FORM_ID_alter().
 *
 */
function rebuild_ajax_form_form_your_custom_id_form_alter(&$form, &$form_state) {
  // Should this form include the JS file to fix the AJAX issue?
  if (isset($form_state['context']['entity_id'])) {
    $nid = $form_state['context']['entity_id'];
    drupal_add_js(array('rebuildAjaxForm' => array(
      'nid' => $nid,
      'form_build_id' => $form['#build_id'],
      ),
    ), 'setting');
    $basepath_mod = drupal_get_path('module', 'rebuild_ajax_form');
    drupal_add_js($basepath_mod . '/rebuild_ajax_form.js', array(
      'weight' => 100,
      ));
  }
}

/**
 * @file rebuild_ajax_form.js
 * Update form_build_id
 */
(function ($, Drupal, window, document, undefined) {
  Drupal.behaviors.rebuildAjaxForm = {
    attach: function (context, settings) {
   /**
     * @see https://www.deeson.co.uk/labs/trigger-drupal-managed-ajax-calls-any-time-drupal-7
     * Add an extra function to the Drupal ajax object
     * which allows us to trigger an ajax response without
     * an element that triggers it.
     */
      Drupal.ajax.prototype.specifiedResponse = function() {
        var ajax = this;
        // Do not perform another ajax command if one is in progress
        if (ajax.ajaxing) {
          return false;
        }
        try {
          $.ajax(ajax.options);
        }
        catch (err) {
          alert("An error occurred in: " + ajax.options.url);
          return false;
        }
        return false;
      };

      // Define a custom ajax action not associated with an element.
      var custom_settings = {};
      custom_settings.url = '/rebuild_ajax_form/' + settings.rebuildAjaxForm.nid;
      custom_settings.event = 'onload';
      custom_settings.keypress = false;
      custom_settings.prevent = false;
      custom_settings.submit = {form_build_id: settings.rebuildAjaxForm.form_build_id};
      Drupal.ajax['custom_ajax_action'] = new Drupal.ajax(null, $(document.body), custom_settings);

      $('form[id|="your-custom-id-form"]', context).once('rebuildAjaxForm', function(){
        Drupal.ajax['custom_ajax_action'].specifiedResponse();
      });
    }
  };
})(jQuery, Drupal, this, this.document);