Dropdown with dynamic content (Ajax/CodeIgniter)

Dropdown with dynamic content (Ajax/CodeIgniter)  

Controller:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Ajax_sample extends CI_Controller {
/* file: application/controllers/ajax_sample.php */

    function index(){
        $data = array();

        $data['arr_field1'] = array(
            1 => 'option 1',
            2 => 'option 2',
            3 => 'option 3',
        );

        $this->load->helper('form');

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

    function get_dropdown_values($filter){

        $this->load->model('test_model');

        $result = $this->test_model->get_options($filter);

        $this->load->helper('form');

        echo form_dropdown('field2', $result, NULL, 'id="field2"');
    }
}

Model:

<?php
class Test_model extends CI_Model {
/* file: application/models/test_model.php */

    function get_options($filter=''){

        switch($filter){
            case 1:
                $arr = array('option A', 'option B', 'option C');
            break;
            case 2:
                $arr = array('option D', 'option E', 'option F');
            break;
            case 3:
                $arr = array('option G', 'option H', 'option I');
            break;
            default: $arr = array('option Z');
        }

        return $arr;
    }
}

View:

<?php

/* file: application/views/sample_dropdown.php */


    echo form_open('insert');

        echo form_dropdown('field1', $arr_field1, NULL, 'id="field1" onchange="load_dropdown_content($(\'#field2\'), this.value)"');

        echo form_dropdown('field2', array('0' => '...'), NULL, 'id="field2"');

    echo form_close();

?>
 
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script type="text/javascript">

function load_dropdown_content(field_dropdown, selected_value){
    var result = $.ajax({
        'url' : '<?php echo site_url('ajax_sample/get_dropdown_values'); ?>/' + selected_value,
        'async' : false
    }).responseText;
    field_dropdown.replaceWith(result);
}

</script>

URL : http://localhost/codeIgniter_3_1_10/ajax_sample