public function validate($messageManager)
 {
     $field = $this->form->getField($this->fieldName);
     $fieldValue = $field->getValue();
     if ($fieldValue) {
         $dao = new UserDAO();
         if (!$dao->isActiveUser($fieldValue)) {
             $messageManager->addMessage('userDoesNotExist', array($this->fieldName => $field->getCaption()));
         }
     }
 }