Database Migration ================== > Note: This section is under development. During the course of developing and maintaining a database-driven application, the structure of the database being used evolves just like the source code does. For example, during the development of an application, a new table may be found necessary; after the application is deployed to production, it may be discovered that an index should be created to improve the query performance; and so on. Because a database structure change often requires some source code changes, Yii supports the so-called *database migration* feature that allows you to keep track of database changes in terms of *database migrations* which are version-controlled together with the source code. The following steps show how database migration can be used by a team during development: 1. Tim creates a new migration (e.g. creates a new table, changes a column definition, etc.). 2. Tim commits the new migration into the source control system (e.g. Git, Mercurial). 3. Doug updates his repository from the source control system and receives the new migration. 4. Doug applies the migration to his local development database, thereby synchronizing his database to reflect the changes that Tim has made. And the following steps show how to deploy a new release with database migrations to production: 1. Scott creates a release tag for the project repository that contains some new database migrations. 2. Scott updates the source code on the production server to the release tag. 3. Scott applies any accumulated database migrations to the production database. Yii provides a set of migration command line tools that allow you to: * create new migrations; * apply migrations; * revert migrations; * re-apply migrations; * show migration history and status. All these tools are accessible through the command `yii migrate`. In this section we will describe in detail how to accomplish various tasks using these tools. You may also get the usage of each tool via the help command `yii help migrate`. ## Creating Migrations To create a new migration, run the following command: ``` yii migrate/create ``` The required `name` argument gives a brief description about the new migration. For example, if the migration is about creating a new table named *news*, you may use the name `create_news_table` and run the following command: ``` yii migrate/create create_news_table ``` > Note: Because the `name` argument will be used as part of the generated migration class name, it should only contain letters, digits, and/or underscore characters. The above command will create a new PHP class file named `m150101_185401_create_news_table.php` in the `@app/migrations` directory. The file contains the following code which mainly declares a migration class `m150101_185401_create_news_table` with the skeleton code: ```php _`, where * `` refers to the UTC datetime when the migration creation command is executed. * `` is the same as the value of the `name` argument that you provide to the command. In the migration class, you are expected to write code in the `up()` method to make changes to the database structure. You may also want to write code in the `down()` method to revert the changes made by the `up()` method. The following code shows how you implement the migration class to create a `news` table: ```php use yii\db\Schema; use yii\db\Migration; class m150101_185401_create_news_table extends \yii\db\Migration { public function up() { $this->createTable('news', [ 'id' => Schema::TYPE_PK, 'title' => Schema::TYPE_STRING . ' NOT NULL', 'content' => Schema::TYPE_TEXT, ]); } public function down() { $this->dropTable('news'); } } ``` > Info: Not all migrations are reversible. For example, if the `up()` method deletes a row of a table, you may not be able to recover this row in the `down()` method. Sometimes, you may be just too lazy to implement the `down()`, because it is not very common to revert database migrations. In this case, you should return `false` in the `down()` method to indicate that the migration is not reversible. The base migration class [[yii\db\Migration]] exposes a database connection via the [[yii\db\Migration::db|db]] property. You can use it to manipulate the database schema using the methods as described in [Working with Database Schema](db-dao.md#database-schema). Rather than using physical types, when creating a table or column you should use *abstract types* so that your migrations are independent of specific DBMS. The [[yii\db\Schema]] class defines a set of constants to represent the supported abstract types. These constants are named in the format of `TYPE_`. For example, `TYPE_PK` refers to auto-incremental primary key type; `TYPE_STRING` refers to a string type. When a migration is applied to a particular database, the abstract types will be translated into the corresponding physical types. In the case of MySQL, `TYPE_PK` will be turned into `int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY`, while `TYPE_STRING` becomes `varchar(255)`. You can append additional constraints when using abstract types. In the above example, ` NOT NULL` is appended to `Schema::TYPE_STRING` to specify that the column cannot be null. > Info: The mapping between abstract types and physical types is specified by the [[yii\db\QueryBuilder::$typeMap|$typeMap]] property in each concrete `QueryBuilder` class. ### Transactional Migrations While performing complex DB migrations, we usually want to make sure that each migration succeeds or fail as a whole so that the database maintains its consistency and integrity. In order to achieve this goal, we can exploit DB transactions. We use the special methods `safeUp` and `safeDown` for these purposes. ```php use yii\db\Schema; class m101129_185401_create_news_table extends \yii\db\Migration { public function safeUp() { $this->createTable('news', [ 'id' => 'pk', 'title' => Schema::TYPE_STRING . ' NOT NULL', 'content' => Schema::TYPE_TEXT, ]); $this->createTable('user', [ 'id' => 'pk', 'login' => Schema::TYPE_STRING . ' NOT NULL', 'password' => Schema::TYPE_STRING . ' NOT NULL', ]); } public function safeDown() { $this->dropTable('news'); $this->dropTable('user'); } } ``` When your code uses more then one query it is recommended to use `safeUp` and `safeDown`. > 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. In the case of MySQL, some SQL statements may cause > [implicit commit](http://dev.mysql.com/doc/refman/5.1/en/implicit-commit.html). 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 you want 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 `migration`. This allows the tool to identify which migrations have been applied and which have not. If the `migration` table does not exist, the tool will automatically create it in the database specified by the `db` [application component](structure-application-components.md). 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 next 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 migration step 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 only the last applied migration will be reverted back. As we described before, not all migrations can be reverted. Trying to revert such migrations will throw an exception and stop the entire 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, which means only the last migration will be redone. 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 a few options that can be specified on the 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 so the migrations are performed as 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 `migration`. The table structure is `version varchar(255) primary key, apply_time integer`. * `db`: string, specifies the ID of the database [application component](structure-application-components.md). 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 a `forum` module whose migration files are located within the module's `migrations` directory, we can use the following command: ``` yii migrate/up --migrationPath=@app/modules/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, ```php 'controllerMap' => [ 'migrate' => [ 'class' => 'yii\console\controllers\MigrateController', 'migrationTable' => 'my_custom_migrate_table', ], ] ``` Now if we run the `migrate` command, the above configurations will take effect without requiring us to enter the command line options every time. Other command options can be also configured this way. ### Migrating with Multiple Databases By default, migrations will be applied to the database specified by the `db` [application component](structure-application-components.md). You may change it by specifying the `--db` option, for example, ``` yii migrate --db=db2 ``` The above command will apply *all* migrations found in the default migration path to the `db2` database. If your application works with multiple databases, it is possible that some migrations should be applied to one database while some others should be applied to another database. In this case, it is recommended that you create a base migration class for each different database and override the [[yii\db\Migration::init()]] method like the following, ```php public function init() { $this->db = 'db2'; parent::init(); } ``` To create a migration that should be applied to a particular database, simply extend from the corresponding base migration class. Now if you run the `yii migrate` command, each migration will be applied to its corresponding database. > Info: Because each migration uses a hardcoded DB connection, the `--db` option of the `migrate` command will have no effect. Also note that the migration history will be stored in the default `db` database. If you want to support changing the DB connection via the `--db` option, you may take the following alternative approach to work with multiple databases. For each database, create a migration path and save all corresponding migration classes there. To apply migrations, run the command as follows, ``` yii migrate --migrationPath=@app/migrations/db1 --db=db1 yii migrate --migrationPath=@app/migrations/db2 --db=db2 ... ``` > Info: The above approach stores the migration history in different databases specified via the `--db` option.