unique username validation in codeigniter code example

Example 1: is_unique in codeigniter form validation

$this->form_validation->set_rules('order_no', 'Order no', 'required|callback_check_order_no');

Example 2: is_unique in codeigniter form validation

function check_unique_order_no($id = '', $order_no) {
        $this->db->where('order_no', $order_no);
        $this->db->where('status', "A");

        if($id) {
            $this->db->where_not_in('id', $id);
        }
        return $this->db->get('delivery_order')->num_rows();
    }