Hello friends

I have a view

echo "<label for='parent'>Category</label><br/> ";
echo form_dropdown('category_id', $categories). "<p>";

controller

function create(){
    if($this->input->post('name')){
        $this->MProducts->addProduct();

        $this->session->set_flashdata('message', 'Products Created');
        redirect('admin/products/index', 'refresh');
    }else{
        $data['title'] = "Create Product";
        $data['main'] = 'admin_product_create';
        $data['categories']= $this->MCats->getTopCategories();
        $this->load->vars($data);
        $this->load->view('dashboard');
    }
}

and the model is

function getTopcategories(){
    $data = array();
    $data[0] = 'root';
    $this->db->where('parentid',0);
    $Q = $this->db->get('categories');
        if($Q->num_rows() > 0){
            foreach($Q->result_array() as $row){
                $data[$row['id']] = $row['name'];
            }
        }
        $Q->free_result();
        return $data;
}

Basically, what i want is we get sub categories when we click categories and 'selected' subcategory id goes to database when we create products, So that we can list products by sub categories. Please help me how do we do that ?