popsu-d7/sites/all/modules/references/references_uuid/references_uuid.module

<?php
/**
 * @file
 *  This provides UUID support for node and user references
 */

/**
 * Implements hook_field_uuid_load().
 */
function node_reference_field_uuid_load($entity_type, $entity, $field, $instance, $langcode, &$items) {
  entity_property_id_to_uuid($items, 'node', 'nid');
}

/**
 * Implements hook_field_uuid_presave().
 */
function node_reference_field_uuid_presave($entity_type, $entity, $field, $instance, $langcode, &$items) {
  entity_property_uuid_to_id($items, 'node', 'nid');
}

/**
 * Implements hook_field_uuid_load().
 */
function user_reference_field_uuid_load($entity_type, $entity, $field, $instance, $langcode, &$items) {
  entity_property_id_to_uuid($items, 'user', 'uid');
}

/**
 * Implements hook_field_uuid_presave().
 */
function user_reference_field_uuid_presave($entity_type, $entity, $field, $instance, $langcode, &$items) {
  entity_property_uuid_to_id($items, 'user', 'uid');
}