.. _app_db_migrations:

==========
Migrations
==========

In the past, apps had a `appinfo/database.xml`-file which holds their database schema
for installation and update and was a functional method for installing apps which
had some trouble with upgrading apps (e.g. apps were not able to rename columns
without losing the data stored in the original column):

.. code-block:: php

   <?xml version="1.0" encoding="ISO-8859-1" ?>
   <database>
            <name>*dbname*</name>
            <create>true</create>
            <overwrite>false</overwrite>
            <charset>utf8</charset>
            <table>
                    <name>*dbprefix*twofactor_backupcodes</name>
                    <declaration>
                          <field>
                                  <name>id</name>
                                  <type>integer</type>
                                  <autoincrement>1</autoincrement>
                                  <default>0</default>
                                  <notnull>true</notnull>
                                  <length>4</length>
                          </field>
    ...


The limitations of this method will be bypassed with migrations. A migration can
consist of 3 different methods:

* Pre schema changes
* Actual 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.

After creating migrations for your current database and installation routine,
you need to in order to make use of migrations, is to delete the old `appinfo/database.xml`
file. The Nextcloud updater logic only allows to use one or the other.
But as soon as the `database.xml` file is gone, it 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:

.. code-block:: php

   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

.. code-block:: php

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

3. Migration 2: Schema change
-----------------------------

With this the old column gets removed.

.. code-block:: php

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

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

          return $schema;
  }

.. _migration_console_command:

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.
* `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.
