/**
  *    _associate_attendee_with_registration
  *
  * @param EE_Registration $registration
  * @param EE_Attendee     $attendee
  * @return void
  */
 private function _associate_attendee_with_registration(EE_Registration $registration, EE_Attendee $attendee)
 {
     // add relation to attendee
     $registration->_add_relation_to($attendee, 'Attendee');
     $registration->set_attendee_id($attendee->ID());
     $registration->update_cache_after_object_save('Attendee', $attendee);
 }