如何在CodeIgniter的迁移中使用down（）方法？

Question

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?

Here's the sample from the documentation

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

class Migration_Add_blog extends CI_Migration {

    public function up()
    {
        $this->dbforge->add_field(array(
            'blog_id' => array(
                'type' => 'INT',
                'constraint' => 5,
                'unsigned' => TRUE,
                'auto_increment' => TRUE
            ),
            'blog_title' => array(
                'type' => 'VARCHAR',
                'constraint' => '100',
            ),
            'blog_description' => array(
                'type' => 'TEXT',
                'null' => TRUE,
            ),
        ));

        $this->dbforge->create_table('blog');
    }

    public function down()
    {
        $this->dbforge->drop_table('blog');
    }

In my page I setup the migration like this:

class Migrate extends CI_Controller {

    public function __construct() {
        parent::__construct();
        $this->load->library('migration');
    }

    public function index() {

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

        if(!$this->migration->current()) {
            show_error($this->migration->error_string());
        } else {
            $data['message'] = 'migrate success';
        }

        renderPage('common/migrate', $data);

    }

}

How can I call the down() method?

Answer 1

tl;dr

Think of Migrations as versions in Git.

Invoke down() method by invoking older migration than current.

ts;dr

Lets say you have 3 migration files as per current time under your directory application/migrations/.

1. 001_add_clients.php
2. 002_add_customers.php
3. 003_add_blog.php

So, as per now your application has passed through 3 migrations as listed above and your current status of migration in your config/migration.php is set to

$config['migration_table'] = 'migrations';
$config['migration_version'] = 3;

Your table named migrations has version set to 3.

Now, when you run $this->migration->current(); through your migration controller, it will do nothing because your are already in current state of migration.

---

Now, somehow your idea of creating blog failed and you want your application to return in migration state 002.

You invoke this by calling $this->migration->version(2); in your controller

or

setting $config['migration_version'] = 2; in your config.php and $this->migration->current(); in your controller.

This is the scenario where

public function down()
{
    $this->dbforge->drop_table('blog');
}

method invokes of file 003_add_blog.php.