title: How to use the down() method in CodeIgniter's migration?
body: <p>I read the CodeIgniter's documentation about the migration class. I am able to perform the migration but how can I use the down() function in the migration?</p>
  
  <p>Here's the sample from the documentation</p>
  
  <pre><code>defined('BASEPATH') OR exit('No direct script access allowed');
  
  class Migration_Add_blog extends CI_Migration {
  
      public function up()
      {
          $this-&gt;dbforge-&gt;add_field(array(
              'blog_id' =&gt; array(
                  'type' =&gt; 'INT',
                  'constraint' =&gt; 5,
                  'unsigned' =&gt; TRUE,
                  'auto_increment' =&gt; TRUE
              ),
              'blog_title' =&gt; array(
                  'type' =&gt; 'VARCHAR',
                  'constraint' =&gt; '100',
              ),
              'blog_description' =&gt; array(
                  'type' =&gt; 'TEXT',
                  'null' =&gt; TRUE,
              ),
          ));
  
          $this-&gt;dbforge-&gt;create_table('blog');
      }
  
      public function down()
      {
          $this-&gt;dbforge-&gt;drop_table('blog');
      }
  </code></pre>
  
  <p>In my page I setup the migration like this:</p>
  
  <pre><code>class Migrate extends CI_Controller {
  
      public function __construct() {
          parent::__construct();
          $this-&gt;load-&gt;library('migration');
      }
  
      public function index() {
  
          $this-&gt;load-&gt;helper('template');
  
          if(!$this-&gt;migration-&gt;current()) {
              show_error($this-&gt;migration-&gt;error_string());
          } else {
              $data['message'] = 'migrate success';
          }
  
          renderPage('common/migrate', $data);
  
      }
  
  }
  </code></pre>
  
  <p>How can I call the down() method?</p>