private function saveFieldMapping(Tracker $mapping_tracker, Tracker_FormElement $field)
 {
     if ($this->dao->save($this->tracker->getId(), $mapping_tracker->getId(), $field->getId())) {
         $this->value_dao->delete($this->tracker->getId(), $mapping_tracker->getId());
         $GLOBALS['Response']->addFeedback('info', $GLOBALS['Language']->getText('plugin_cardwall', 'on_top_mapping_changed', array($mapping_tracker->getName(), $field->getLabel())));
     }
 }