gistfile1.txt

/**
 * Implements hook_entity_insert().
 */
function hook_entity_insert($entity, $type) {
  $bundle = isset($entity->type) ? $entity->type : '*';

  // new entity
  if (isset($entity->is_new) && $entity->is_new) {
    if (isset($_GET['id']) && is_numeric($_GET['id']) && isset($_GET['type'])) {
      $target_id = intval($_GET['id']);
      $target_type = $_GET['type'];

      // Add join with parent entity
      switch ("{$type}:{$bundle}") {
        case 'departure:departure':
          $wrapper = entity_metadata_wrapper($target_type, $target_id);
          if (isset($wrapper->field_ref_departures)) {
            $wrapper->field_ref_departures[] = $entity->id;
          }
          $wrapper->save();
          break;
      }
    }
  }

  crm__entity_save($entity, $type);
}