Documentations

Database Migrations Documentation CodeIgniter 3.x <http://codeigniter.com/userguide3/libraries/migration.html>_
:doc:Database Migrations Documentation CodeIgniter 4.x </dbmgmt/migration>

What has been changed

First of all, the sequential naming (001_create_users, 002_create_posts) of migrations is not longer supported. Version 4 of CodeIgniter only supports the timestamp scheme (20121031100537_create_users, 20121031500638_create_posts) . If you have used sequential naming you have to rename each migration file.
The migration table definition was changed. If you upgrade from CI3 to CI4 and use the same database, You need to upgrade the migration table definition and its data.
The migration procedure has been also changed. You can now migrate the database with a simple CLI command:

.. code-block:: console

php spark migrate

If your v3 project uses sequential migration names you have to change those to timestamp names.
You have to move all migration files to the new folder app/Database/Migrations.
Remove the line defined('BASEPATH') OR exit('No direct script access allowed'); if it exists.
Add this line just after the opening php tag: namespace App\Database\Migrations;.
Below the namespace App\Database\Migrations; line add this line: use CodeIgniter\Database\Migration;
Replace extends CI_Migration with extends Migration.
The method names within the Forge class has been changed to use camelCase. For example:
- $this->dbforge->add_field to $this->forge->addField
- $this->dbforge->add_key to $this->forge->addKey
- $this->dbforge->create_table to $this->forge->addTable
- $this->dbforge->drop_table to $this->forge->addTable
(optional) You can change the array syntax from array(...) to [...]
Upgrade the migration table, if you use the same database.
- (development) Run the CI4 migration in the development environment or so with brand new database, to create the new migration table.
- (development) Export the migration table.
- (production) Drop (or rename) the existing CI3 migration table.
- (production) Import the new migration table and the data.

Path: application/migrations:

.. literalinclude:: upgrade_migrations/ci3sample/001.php

Path: app/Database/Migrations:

.. literalinclude:: upgrade_migrations/001.php

You can use to following table to search & replace your old CI3 files.