How to create dropdown select from db with Codeigniter

Your Database Structure

            Cteate table as tb_category:

   - category_id
   - category_name

Controller

class category extends CI_Controller {

   public function __construct()
   {
       parent::__construct();
       $this->load->model('category_model');

       // Load session library
       $this->load->library('session');

   }

   public function index()
   {
       $data['value'] = $this->category_model->get_data();

       $this->load->view('category',$data );
   }
}

Model

class category_model extends CI_Model {

public function __construct()
    {
        parent::__construct();
    }

public function get_data_national()
    {
        //Select data and query from table
        $this->db->select('*');
        $this->db->from('tb_category');
        $query = $this->db->get();
        $result = $query->result();

        //store data into array format
        $id = array('');
        $name = array('...Select Category...');

        //Loop data from table
        for ($i = 0; $i < count($result); $i++)
        {
            array_push($id, $result[$i]->id);
            array_push($name, $result[$i]->nationality);
        }

        //Loading data into value and name for select box
        return $getData = array_combine($id,$name);
    }
}

View

    <?php
         $attributes = 'id="levels" style="width: 315px;" class="chzn-select"'; // select box style
        echo form_dropdown('category', $value, set_value('category'),$attributes);
?>