public function validate($messageManager)
 {
     parent::validate($messageManager);
     if (!$messageManager->isAnyErrorMessage()) {
         $recordId = $this->form->getField($this->idFieldName)->getValue();
         $field = $this->form->getField($this->fieldName);
         $fieldValue = $field->getValue();
         if (empty($recordId) || $fieldValue != $this->oldValue) {
             if (!empty($fieldValue)) {
                 $dao = new UserDAO();
                 if ($dao->isRegisteredUser($fieldValue)) {
                     $messageManager->addMessage('userEmailAlreadyRegistered', array($this->fieldName => $field->getCaption()));
                 }
             }
         }
     }
 }