public function saveEntity($identifier, FormData $requestData, $subResource = NULL) {
    $revision = $this->defaultRevision;
    $entity = $this->getEntity($identifier);
    
    if ($subResource !== NULL && array_key_exists($subResource, $this->customActions)) {
      return $this->callCustomAction($this->customActions[$subResource], array($entity, $requestData, $revision));
    }
    
    return $this->updateEntityRevision($entity, $revision, $requestData, $subResource);
  }