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Database Migration

Like source code, the structure of a database is evolving as we develop and maintain a database-driven application. For example, during development, we may want to add a new table; or after the application is put into production, we may realize the need of adding an index on a column. It is important to keep track of these structural database changes (called migration) like we do with our source code. If the source code and the database are out of sync, it is very likely the whole system may break. For this reason, Yii provides a database migration tool that can keep track of database migration history, apply new migrations, or revert existing ones.

The following steps show how we can use database migration during development:

Tim creates a new migration (e.g. create a new table)
Tim commits the new migration into source control system (e.g. GIT, Mercurial)
Doug updates from source control system and receives the new migration
Doug applies the migration to his local development database

Yii supports database migration via the yii migrate command line tool. This tool supports creating new migrations, applying/reverting/redoing migrations, and showing migration history and new migrations.

Creating Migrations

To create a new migration (e.g. create a news table), we run the following command:

yii migrate/create <name>

The required name parameter specifies a very brief description of the migration (e.g. create_news_table). As we will show in the following, the name parameter is used as part of a PHP class name. Therefore, it should only contain letters, digits and/or underscore characters.

yii migrate/create create_news_table

The above command will create under the protected/migrations directory a new file named m101129_185401_create_news_table.php which contains the following initial code:

class m101129_185401_create_news_table extends \yii\db\Migration
{
	public function up()
	{
	}

	public function down()
	{
		echo "m101129_185401_create_news_table cannot be reverted.\n";
		return false;
	}
}

Notice that the class name is the same as the file name which is of the pattern m<timestamp>_<name>, where <timestamp> refers to the UTC timestamp (in the format of yymmdd_hhmmss) when the migration is created, and <name> is taken from the command's name parameter.

The up() method should contain the code implementing the actual database migration, while the down() method may contain the code reverting what is done in up().

Sometimes, it is impossible to implement down(). For example, if we delete table rows in up(), we will not be able to recover them in down(). In this case, the migration is called irreversible, meaning we cannot roll back to a previous state of the database. In the above generated code, the down() method returns false to indicate that the migration cannot be reverted.

As an example, let's show the migration about creating a news table.

class m101129_185401_create_news_table extends \yii\db\Migration
{
	public function up()
	{
		$this->db->createCommand()->createTable('tbl_news', array(
			'id' => 'pk',
			'title' => 'string(128) NOT NULL',
			'content' => 'text',
		))->execute();
	}

	public function down()
	{
		$this->db->createCommand()->dropTable('tbl_news')->execute();
	}
}

The base class [\yii\db\Migration] exposes a database connection via db property. You can use it for manipulating data and schema of a database.

The column types used in this example are abstract types that will be replaced by Yii with the corresponding types depended on your database management system. You can use them to write database independent migrations. For example pk will be replaced by int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY for MySQL and integer PRIMARY KEY AUTOINCREMENT NOT NULL for sqlite. See documentation of QueryBuilder::getColumnType() for more details and a list of available types. You may also use the constants defined in \yii\db\Schema to define column types.

Transactional Migrations

While performing complex DB migrations, we usually want to make sure that each migration succeed or fail as a whole so that the database maintains the consistency and integrity. In order to achieve this goal, we can exploit DB transactions.

We could explicitly start a DB transaction and enclose the rest of the DB-related code within the transaction, like the following:

class m101129_185401_create_news_table extends \yii\db\Migration
{
	public function up()
	{
		$transaction=$this->getDbConnection()->beginTransaction();
		try
		{
			$this->db->createCommand()->createTable('tbl_news', array(
				'id' => 'pk',
				'title' => 'string NOT NULL',
				'content' => 'text',
			))->execute();
			$transaction->commit();
		}
		catch(Exception $e)
		{
			echo "Exception: ".$e->getMessage()."\n";
			$transaction->rollback();
			return false;
		}
	}

	// ...similar code for down()
}

Note: Not all DBMS support transactions. And some DB queries cannot be put into a transaction. In this case, you will have to implement up() and down(), instead. And for MySQL, some SQL statements may cause implicit commit.

Applying Migrations

To apply all available new migrations (i.e., make the local database up-to-date), run the following command:

yii migrate

The command will show the list of all new migrations. If you confirm to apply the migrations, it will run the up() method in every new migration class, one after another, in the order of the timestamp value in the class name.

After applying a migration, the migration tool will keep a record in a database table named tbl_migration. This allows the tool to identify which migrations have been applied and which are not. If the tbl_migration table does not exist, the tool will automatically create it in the database specified by the db application component.

Sometimes, we may only want to apply one or a few new migrations. We can use the following command:

yii migrate/up 3

This command will apply the 3 new migrations. Changing the value 3 will allow us to change the number of migrations to be applied.

We can also migrate the database to a specific version with the following command:

yii migrate/to 101129_185401

That is, we use the timestamp part of a migration name to specify the version that we want to migrate the database to. If there are multiple migrations between the last applied migration and the specified migration, all these migrations will be applied. If the specified migration has been applied before, then all migrations applied after it will be reverted (to be described in the next section).

Reverting Migrations

To revert the last one or several applied migrations, we can use the following command:

yii migrate/down [step]

where the optional step parameter specifies how many migrations to be reverted back. It defaults to 1, meaning reverting back the last applied migration.

As we described before, not all migrations can be reverted. Trying to revert such migrations will throw an exception and stop the whole reverting process.

Redoing Migrations

Redoing migrations means first reverting and then applying the specified migrations. This can be done with the following command:

yii migrate/redo [step]

where the optional step parameter specifies how many migrations to be redone. It defaults to 1, meaning redoing the last migration.

Showing Migration Information

Besides applying and reverting migrations, the migration tool can also display the migration history and the new migrations to be applied.

yii migrate/history [limit]
yii migrate/new [limit]

where the optional parameter limit specifies the number of migrations to be displayed. If limit is not specified, all available migrations will be displayed.

The first command shows the migrations that have been applied, while the second command shows the migrations that have not been applied.

Modifying Migration History

Sometimes, we may want to modify the migration history to a specific migration version without actually applying or reverting the relevant migrations. This often happens when developing a new migration. We can use the following command to achieve this goal.

yii migrate/mark 101129_185401

This command is very similar to yii migrate/to command, except that it only modifies the migration history table to the specified version without applying or reverting the migrations.

Customizing Migration Command

There are several ways to customize the migration command.

Use Command Line Options

The migration command comes with four options that can be specified in command line:

interactive: boolean, specifies whether to perform migrations in an interactive mode. Defaults to true, meaning the user will be prompted when performing a specific migration. You may set this to false should the migrations be done in a background process.
migrationPath: string, specifies the directory storing all migration class files. This must be specified in terms of a path alias, and the corresponding directory must exist. If not specified, it will use the migrations sub-directory under the application base path.
migrationTable: string, specifies the name of the database table for storing migration history information. It defaults to tbl_migration. The table structure is version varchar(255) primary key, apply_time integer.
connectionID: string, specifies the ID of the database application component. Defaults to 'db'.
templateFile: string, specifies the path of the file to be served as the code template for generating the migration classes. This must be specified in terms of a path alias (e.g. application.migrations.template). If not set, an internal template will be used. Inside the template, the token {ClassName} will be replaced with the actual migration class name.

To specify these options, execute the migrate command using the following format

yii migrate/up --option1=value1 --option2=value2 ...

For example, if we want to migrate for a forum module whose migration files are located within the module's migrations directory, we can use the following command:

yii migrate/up --migrationPath=ext.forum.migrations

Configure Command Globally

While command line options allow us to configure the migration command on-the-fly, sometimes we may want to configure the command once for all. For example, we may want to use a different table to store the migration history, or we may want to use a customized migration template. We can do so by modifying the console application's configuration file like the following,

TBD

Now if we run the migrate command, the above configurations will take effect without requiring us to enter the command line options every time.

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