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()));
}
}
}