user_guide_src/source/libraries/migration.rst

################
Migrations Class
################

Migrations are a convenient way for you to alter your database in a 
structured and organized manner. You could edit fragments of SQL by hand 
but you would then be responsible for telling other developers that they 
need to go and run them. You would also have to keep track of which changes 
need to be run against the production machines next time you deploy.

The database table **migration** tracks which migrations have already been 
run so all you have to do is update your application files and 
call **$this->migrate->current()** to work out which migrations should be run. 
The current version is found in **config/migration.php**.

********************
Migration file names
********************

Each Migration is run in numeric order forward or backwards depending on the
method taken. Two numbering styles are available:

* **Sequential:** each migration is numbered in sequence, starting with **001**.
  Each number must be three digits, and there must not be any gaps in the
  sequence. (This was the numbering scheme prior to CodeIgniter 3.0.)
* **Timestamp:** each migration is numbered using the timestamp when the migration
  was created, in **YYYYMMDDHHIISS** format (e.g. **20121031100537**). This
  helps prevent numbering conflicts when working in a team environment, and is
  the preferred scheme in CodeIgniter 3.0 and later.

The desired style may be selected using the **$config['migration_type']**
setting in your **migration.php** config file.

Regardless of which numbering style you choose to use, prefix your migration
files with the migration number followed by an underscore and a descriptive
name for the migration. For example:

* **001_add_blog.php** (sequential numbering)
* **20121031100537_add_blog.php** (timestamp numbering)

******************
Create a Migration
******************
	
This will be the first migration for a new site which has a blog. All 
migrations go in the folder **application/migrations/** and have names such 
as **20121031100537_add_blog.php**.::

	<?php

	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->add_key('blog_id', TRUE);
			$this->dbforge->create_table('blog');
		}

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

Then in **application/config/migration.php** set **$config['migration_version'] = 1;**.

*************
Usage Example
*************

In this example some simple code is placed in **application/controllers/migrate.php** 
to update the schema.::

	<?php
	
	class Migrate extends CI_Controller
	{
	    public function index()
	    {
	    	$this->load->library('migration');
	    	
	    	if ($this->migration->current() === FALSE)
	    	{
	    		show_error($this->migration->error_string());
	    	}
	    }
	}

******************
Function Reference
******************

$this->migration->current()
============================

The current migration is whatever is set for **$config['migration_version']** in 
**application/config/migration.php**.

$this->migration->error_string()
=================================

This returns a string of errors while performing a migration.

$this->migration->find_migrations()
====================================

An array of migration filenames are returned that are found in the **migration_path** 
property.

$this->migration->latest()
===========================

This works much the same way as current() but instead of looking for 
the **$config['migration_version']** the Migration class will use the very 
newest migration found in the filesystem.

$this->migration->version()
============================

Version can be used to roll back changes or step forwards programmatically to 
specific versions. It works just like current but ignores **$config['migration_version']**.::

	$this->load->library('migration');

	$this->migration->version(5);

*********************
Migration Preferences
*********************

The following is a table of all the config options for migrations.

========================== ====================== ========================== =============================================
Preference                 Default                Options                    Description
========================== ====================== ========================== =============================================
**migration_enabled**      FALSE                  TRUE / FALSE               Enable or disable migrations.
**migration_path**         APPPATH.'migrations/'  None                       The path to your migrations folder.
**migration_version**      0                      None                       The current version your database should use.
**migration_table**        migrations             None                       The table name for storing the schema
                                                                             version number.
**migration_auto_latest**  FALSE                  TRUE / FALSE               Enable or disable automatically 
                                                                             running migrations.
**migration_type**         'timestamp'            'timestamp' / 'sequential' The type of numeric identifier used to name
                                                                             migration files.
========================== ====================== ========================== =============================================