Migrations

Migrations change the database schema and operate in three steps:

Pre schema changes
Schema changes
Post schema changes

Apps can have multiple migrations, which allows a way more flexible updating process. For example, you can rename a column while copying all the content with 3 steps packed in 2 migrations.

The Nextcloud updater logic will look for your migration files in the apps lib/Migration folder.

Note

While in theory you can run any code in the pre- and post-steps, we recommend not to use actual php classes. With migrations you can update from any old version to any new version as long as the migration steps are retained. Since they are also used for installation, you should keep them anyway. But this also means when you change a php class which you use in your migration, the code may be executed on different database/file/code standings when being ran in an upgrade situation.

Note

Since Nextcloud stores, which migrations have been executed already you must not “update” migrations. The recommendation is to keep them untouched as long as possible. You should only adjust it to make sure it still executes, but additional changes to the database should be done in a new migration.

1. Migration 1: Schema change

With this step the new column gets created:

public function changeSchema(IOutput $output, \Closure $schemaClosure, array $options) {
                   /** @var ISchemaWrapper $schema */
                   $schema = $schemaClosure();

                   $table = $schema->getTable('twofactor_backupcodes');

                   $table->addColumn('user_id', \OCP\DB\Types::STRING, [
                           'notnull' => true,
                           'length' => 64,
                   ]);

                   return $schema;
      }

2. Migration 1: Post schema change

In this step the content gets copied from the old to the new column.

Note

This could also be done as part of the second migration as part of a pre schema change

public function postSchemaChange(IOutput $output, \Closure $schemaClosure, array $options) {
       $query = $this->db->getQueryBuilder();
       $query->update('twofactor_backupcodes')
               ->set('user_id', 'uid');
       $query->executeStatement();
}

3. Migration 2: Schema change

With this the old column gets removed.

 public function changeSchema(IOutput $output, \Closure $schemaClosure, array $options) {
        /** @var ISchemaWrapper $schema */
        $schema = $schemaClosure();

        $table = $schema->getTable('twofactor_backupcodes');
        $table->dropColumn('uid');

        return $schema;
}

Construction of migration classes

All migration classes are constructed via Dependency injection. So if your migration steps need additional dependencies, these can be defined in the constructor of your migration class.

Example: If your migration needs to execute SQL statements, inject a OCP\IDBConnection instance into your migration class like this:

class Version2404Date20220903071748 extends SimpleMigrationStep {

   /** @var IDBConnection */
   private $db;

   public function __construct(IDBConnection $db) {
      $this->db = $db;
   }

   public function postSchemaChange(IOutput $output, \Closure $schemaClosure, array $options) {
      $query = $this->db->getQueryBuilder();
      // execute some SQL ...
   }
}

Console commands

There are some console commands, which should help developers to create or deal with migrations, which are only available if you are running your Nextcloud in debug mode:

migrations:execute: Executes a single migration version manually. The version argument is the class name of the migration, without the “Version” prefix. For example if your migration was named Version2404Date20220903071748 the version would be 2404Date20220903071748.
migrations:generate: This is needed to create a new migration file. This takes 2 arguments, first one is the appid, the second one should be the version`of your app as an integer. We recommend to use the major and minor digits of your apps version for that. This allows you to introduce a new migration in your branch for a Nextcloud version if there is already a migration path for a newer one in another branch. Since you can’t change this retroactive, we recommend to leave enough space in between and therefore map the numbers to 3 digits: `1.0.x => 1000, 2.34.x => 2034, etc.
migrations:migrate: Execute a migration to a specified or the latest available version.
migrations:status: View the status of a set of migrations.

Note

After generating a migration, you might need to run composer dump-autoload to be able to execute it.

Adding indices

Adding indices to existing tables can take long time, especially on large tables. Therefore it is recommended to not add the indices in the migration itself, but to indicate the index requirement to the server by adding a listener for the AddMissingIndicesEvent. This way the migration can be executed in a separate step and do not block the upgrade process. For new installations the index should still be added to the migration that creates the table.

class AddMissingIndicesListener implements IEventListener {
   public function handle(Event $event): void {
      if (!$event instanceof AddMissingIndicesEvent) {
         return;
      }

      $event->addMissingIndex('my_table', 'my_index', ['column_a', 'column_b']);
   }
}