Unique Check in Codeigniter

Unique Check in Codeigniter::



$this->form_validation->set_rules('email', 'email', 'trim|required|valid_email');


//Customized Unique Check::

if($this->input->post('user_name') != $original_value) {
   $is_unique =  '|is_unique[users.user_name]'
} else {
   $is_unique =  ''
}

$this->form_validation->set_rules('user_name', 'User Name', 'required|trim|xss_clean'.$is_unique);