Tutorial: Migrations¶

Introduction¶

This tutorial explains:

Migrations: Database changes

Migrations¶

Migrations are a means of facilitating database changes, much in the same way UMIL was used in phpBB 3.0. Through Migrations, extension developers are able to easily modify and add new schema and data to the database as required by an extension.

Read the Migrations: Getting started Developer Docs for in-depth migration documentation. We will briefly summarise the key concepts of migrations here.

Database changes¶

Schema changes¶

The update_schema() method is for facilitating schema changes, such as adding new tables, columns, keys and indexes.

The revert_schema() method should always be including to undo any changes introduced by the update_schema() method.

We recommend putting schema changes in their own migration.

Learn more: Schema changes.

Data changes¶

The update_data() method is for inserting, updating and dropping field data.

The revert_data() method is optional and can be used to undo data changes. Normally, the update_data() method is automatically reverted during a purge step, reversing any of the changes introduced by the migration tools. Using the revert_data() method is only needed to perform additional changes during an extension’s uninstall, such as performing a custom functions.

We recommend putting data changes in their own migration.

Learn more: Data changes.

Migration tools¶

Config Tool: The config tool helps adding, updating, and removing config items.
Config Text Tool: The config_text tool helps adding, updating, and removing config_text items. The config_text table is used to store options with an arbitrary length value in a TEXT column. In contrast to config values, config_text are not cached or prefetched.
Module Tool: The module tool helps adding and removing ACP, MCP, and UCP modules.
Permission Tool: The permission tool helps adding, removing, setting, and unsetting permissions and adding or removing permission roles.

Migration dependencies¶

depends_on()¶

The depends_on() method is used to define a migration’s dependencies. Dependencies tell the migrator what order migrations must be installed in.

Learn more: Dependencies.

effectively_installed()¶

The effectively_installed() method is used primarily to help transition from a previous database installer method (such as a MOD that used UMIL) to migrations. However, we recommend using it all the time to ensure safer migrations.

When effectively_installed returns true, the migration is deemed to already have been installed, meaning the migration will be skipped. This helps prevent rewriting (or overwriting) existing changes to the database that may have already been put in place by a previous migration or MOD installation.

For example, in the following code the migration would not be applied if the specified config value already exists in the database:

public function effectively_installed()
{
   return isset($this->config['acme_demo_goodbye']);
}

Warning

As a general rule, migrations should never be changed once they have been installed. Changing a migration in an extension that has already been installed could prevent it from reverting database changes or uninstalling successfully. In order to make subsequent changes to the database, new migrations should be created to implement the additional changes or updates. The only exceptions to this rule are to fix SQL and logic errors or bugs, as long as they do not alter the database changes that have already been made or the dependencies listed in the migration’s depends_on() method.

Once you are familiar with how Migrations work, continue on to the next section to learn how to create and install an ACP module that will allow us to configure some settings for the Acme Demo extension.