DataMapper CodeIgniter - Dropdown list

[eluser]Unknown[/eluser]
I want to have a dropdown list with brands, and depending on the choice, it will appear the products of the brand selected.

My Tables

Code:

Brands
==================
id   |   Brand
---------------
1        Apple
2        Android

Code:

Products
========================================================================
id |    Name           |  Price |  Image      |  Category | Brand_id
------------------------------------------------------------------------------
1    Macbook Air          1000    mac1.jpg        computer       1
2    Macbook Pro          1500    mac2.jpg       computer        1
3    Android S3           400      s3.jpg          mobile        2

My Models

Brands.php

Code:

<?
        class Brands extends DataMapper {
           var $table = 'brands';
           var $has_many = array('Products');
        }
    ?>

Products.php

Code:

<?
        class Products extends DataMapper {
            var $table = 'products';
            var $has_one = array('Brands');
        }
    ?>

My Controller

ListBrands.php

Code:

<?

class ListBrands extends CI_Controller {
    function __construct() {
        parent::__construct();
    }

    function index() {
        $this->load->view('listbrands_view');
    }
}
?>

I don't know if my controller or models are correctly and also I don't know how to do what I'm asking for, a dropdown that will print the products of the brand selected.