# Database Migration ¶

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`.

> Tip: Migrations could affect not only database schema but adjust existing data to fit new schema, create RBAC hierarchy or clean up cache.

> Note: When manipulating data using a migration you may find that using your Active Record classes for this might be useful because some of the logic is already implemented there. Keep in mind however, that in contrast to code written in the migrations, who's nature is to stay constant forever, application logic is subject to change. So when using Active Record in migration code, changes to the logic in the Active Record layer may accidentally break existing migrations. For this reason migration code should be kept independent from other application logic such as Active Record classes.

## Creating Migrations ¶

To create a new migration, run the following command:

    yii migrate/create <name>

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

    use yii\db\Migration;

    class m150101_185401_create_news_table extends Migration
    {
        public function up()
        {

        }

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

            return false;
        }

        /*
        // Use safeUp/safeDown to run migration code within a transaction
        public function safeUp()
        {
        }

        public function safeDown()
        {
        }
        */
    }

Each database migration is defined as a PHP class extending from yii\db\Migration. The migration class name is automatically generated in the format of `m<YYMMDD_HHMMSS>_<Name>`, where

- `<YYMMDD_HHMMSS>` refers to the UTC datetime at which the migration creation command is executed.
- `<Name>` 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 that makes changes to the database structure. You may also want to write code in the `down()` method to revert the changes made by `up()`. The `up()` method is invoked when you upgrade the database with this migration, while the `down()` method is invoked when you downgrade the database. The following code shows how you may 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 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 db property. You can use it to manipulate the database schema using the methods as described in Working with 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_<Name>`. 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 $typeMap property in each concrete `QueryBuilder` class.

Since version 2.0.6, you can make use of the newly introduced schema builder which provides more convenient way of defining column schema. So the migration above could be written like the following:

    <?php

    use yii\db\Migration;

    class m150101_185401_create_news_table extends Migration
    {
        public function up()
        {
            $this->createTable('news', [
                'id' => $this->primaryKey(),
                'title' => $this->string()->notNull(),
                'content' => $this->text(),
            ]);
        }

        public function down()
        {
            $this->dropTable('news');
        }
    }

A list of all available methods for defining the column types is available in the API documentation of yii\db\SchemaBuilderTrait.

## Generating Migrations ¶

Since version 2.0.7 migration console provides a convenient way to create migrations.

If the migration name is of a special form, for example `create_xxx_table` or `drop_xxx_table` then the generated migration file will contain extra code, in this case for creating/dropping tables. In the following all variants of this feature are described.

### Create Table ¶

    yii migrate/create create_post_table

generates

    /**
     * Handles the creation for table `post`.
     */
    class m150811_220037_create_post_table extends Migration
    {
        /**
         * {@inheritdoc}
         */
        public function up()
        {
            $this->createTable('post', [
                'id' => $this->primaryKey()
            ]);
        }

        /**
         * {@inheritdoc}
         */
        public function down()
        {
            $this->dropTable('post');
        }
    }

To create table fields right away, specify them via `--fields` option.

    yii migrate/create create_post_table --fields="title:string,body:text"

generates

    /**
     * Handles the creation for table `post`.
     */
    class m150811_220037_create_post_table extends Migration
    {
        /**
         * {@inheritdoc}
         */
        public function up()
        {
            $this->createTable('post', [
                'id' => $this->primaryKey(),
                'title' => $this->string(),
                'body' => $this->text(),
            ]);
        }

        /**
         * {@inheritdoc}
         */
        public function down()
        {
            $this->dropTable('post');
        }
    }

You can specify more field parameters.

    yii migrate/create create_post_table --fields="title:string(12):notNull:unique,body:text"

generates

    /**
     * Handles the creation for table `post`.
     */
    class m150811_220037_create_post_table extends Migration
    {
        /**
         * {@inheritdoc}
         */
        public function up()
        {
            $this->createTable('post', [
                'id' => $this->primaryKey(),
                'title' => $this->string(12)->notNull()->unique(),
                'body' => $this->text()
            ]);
        }

        /**
         * {@inheritdoc}
         */
        public function down()
        {
            $this->dropTable('post');
        }
    }

> Note: Primary key is added automatically and is named `id` by default. If you want to use another name you may specify it explicitly like `--fields="name:primaryKey"`.

#### Foreign keys ¶

Since 2.0.8 the generator supports foreign keys using the `foreignKey` keyword.

    yii migrate/create create_post_table --fields="author_id:integer:notNull:foreignKey(user),category_id:integer:defaultValue(1):foreignKey,title:string,body:text"

generates

    /**
     * Handles the creation for table `post`.
     * Has foreign keys to the tables:
     *
     * - `user`
     * - `category`
     */
    class m160328_040430_create_post_table extends Migration
    {
        /**
         * {@inheritdoc}
         */
        public function up()
        {
            $this->createTable('post', [
                'id' => $this->primaryKey(),
                'author_id' => $this->integer()->notNull(),
                'category_id' => $this->integer()->defaultValue(1),
                'title' => $this->string(),
                'body' => $this->text(),
            ]);

            // creates index for column `author_id`
            $this->createIndex(
                'idx-post-author_id',
                'post',
                'author_id'
            );

            // add foreign key for table `user`
            $this->addForeignKey(
                'fk-post-author_id',
                'post',
                'author_id',
                'user',
                'id',
                'CASCADE'
            );

            // creates index for column `category_id`
            $this->createIndex(
                'idx-post-category_id',
                'post',
                'category_id'
            );

            // add foreign key for table `category`
            $this->addForeignKey(
                'fk-post-category_id',
                'post',
                'category_id',
                'category',
                'id',
                'CASCADE'
            );
        }

        /**
         * {@inheritdoc}
         */
        public function down()
        {
            // drops foreign key for table `user`
            $this->dropForeignKey(
                'fk-post-author_id',
                'post'
            );

            // drops index for column `author_id`
            $this->dropIndex(
                'idx-post-author_id',
                'post'
            );

            // drops foreign key for table `category`
            $this->dropForeignKey(
                'fk-post-category_id',
                'post'
            );

            // drops index for column `category_id`
            $this->dropIndex(
                'idx-post-category_id',
                'post'
            );

            $this->dropTable('post');
        }
    }

The position of the `foreignKey` keyword in the column description doesn't change the generated code. That means:

- `author_id:integer:notNull:foreignKey(user)`
- `author_id:integer:foreignKey(user):notNull`
- `author_id:foreignKey(user):integer:notNull`

All generate the same code.

The `foreignKey` keyword can take a parameter between parenthesis which will be the name of the related table for the generated foreign key. If no parameter is passed then the table name will be deduced from the column name.

In the example above `author_id:integer:notNull:foreignKey(user)` will generate a column named `author_id` with a foreign key to the `user` table while `category_id:integer:defaultValue(1):foreignKey` will generate a column `category_id` with a foreign key to the `category` table.

Since 2.0.11, `foreignKey` keyword accepts a second parameter, separated by whitespace. It accepts the name of the related column for the foreign key generated. If no second parameter is passed, the column name will be fetched from table schema. If no schema exists, primary key isn't set or is composite, default name `id` will be used.

### Drop Table ¶

    yii migrate/create drop_post_table --fields="title:string(12):notNull:unique,body:text"

generates

    class m150811_220037_drop_post_table extends Migration
    {
        public function up()
        {
            $this->dropTable('post');
        }

        public function down()
        {
            $this->createTable('post', [
                'id' => $this->primaryKey(),
                'title' => $this->string(12)->notNull()->unique(),
                'body' => $this->text()
            ]);
        }
    }

### Add Column ¶

If the migration name is of the form `add_xxx_column_to_yyy_table` then the file content would contain `addColumn` and `dropColumn` statements necessary.

To add column:

    yii migrate/create add_position_column_to_post_table --fields="position:integer"

generates

    class m150811_220037_add_position_column_to_post_table extends Migration
    {
        public function up()
        {
            $this->addColumn('post', 'position', $this->integer());
        }

        public function down()
        {
            $this->dropColumn('post', 'position');
        }
    }

You can specify multiple columns as follows:

    yii migrate/create add_xxx_column_yyy_column_to_zzz_table --fields="xxx:integer,yyy:text"

### Drop Column ¶

If the migration name is of the form `drop_xxx_column_from_yyy_table` then the file content would contain `addColumn` and `dropColumn` statements necessary.

    yii migrate/create drop_position_column_from_post_table --fields="position:integer"

generates

    class m150811_220037_drop_position_column_from_post_table extends Migration
    {
        public function up()
        {
            $this->dropColumn('post', 'position');
        }

        public function down()
        {
            $this->addColumn('post', 'position', $this->integer());
        }
    }

### Add Junction Table ¶

If the migration name is of the form `create_junction_table_for_xxx_and_yyy_tables` or `create_junction_xxx_and_yyy_tables` then code necessary to create junction table will be generated.

    yii migrate/create create_junction_table_for_post_and_tag_tables --fields="created_at:dateTime"

generates

    /**
     * Handles the creation for table `post_tag`.
     * Has foreign keys to the tables:
     *
     * - `post`
     * - `tag`
     */
    class m160328_041642_create_junction_table_for_post_and_tag_tables extends Migration
    {
        /**
         * {@inheritdoc}
         */
        public function up()
        {
            $this->createTable('post_tag', [
                'post_id' => $this->integer(),
                'tag_id' => $this->integer(),
                'created_at' => $this->dateTime(),
                'PRIMARY KEY(post_id, tag_id)',
            ]);

            // creates index for column `post_id`
            $this->createIndex(
                'idx-post_tag-post_id',
                'post_tag',
                'post_id'
            );

            // add foreign key for table `post`
            $this->addForeignKey(
                'fk-post_tag-post_id',
                'post_tag',
                'post_id',
                'post',
                'id',
                'CASCADE'
            );

            // creates index for column `tag_id`
            $this->createIndex(
                'idx-post_tag-tag_id',
                'post_tag',
                'tag_id'
            );

            // add foreign key for table `tag`
            $this->addForeignKey(
                'fk-post_tag-tag_id',
                'post_tag',
                'tag_id',
                'tag',
                'id',
                'CASCADE'
            );
        }

        /**
         * {@inheritdoc}
         */
        public function down()
        {
            // drops foreign key for table `post`
            $this->dropForeignKey(
                'fk-post_tag-post_id',
                'post_tag'
            );

            // drops index for column `post_id`
            $this->dropIndex(
                'idx-post_tag-post_id',
                'post_tag'
            );

            // drops foreign key for table `tag`
            $this->dropForeignKey(
                'fk-post_tag-tag_id',
                'post_tag'
            );

            // drops index for column `tag_id`
            $this->dropIndex(
                'idx-post_tag-tag_id',
                'post_tag'
            );

            $this->dropTable('post_tag');
        }
    }

Since 2.0.11 foreign key column names for junction tables are fetched from table schema. In case table isn't defined in schema, or the primary key isn't set or is composite, default name `id` is used.

### Transactional Migrations ¶

While performing complex DB migrations, it is important to ensure each migration to either succeed or fail as a whole so that the database can maintain integrity and consistency. To achieve this goal, it is recommended that you enclose the DB operations of each migration in a transaction.

An even easier way of implementing transactional migrations is to put migration code in the `safeUp()` and `safeDown()` methods. These two methods differ from `up()` and `down()` in that they are enclosed implicitly in a transaction. As a result, if any operation in these methods fails, all prior operations will be rolled back automatically.

In the following example, besides creating the `news` table we also insert an initial row into this table.

    <?php

    use yii\db\Migration;

    class m150101_185401_create_news_table extends Migration
    {
        public function safeUp()
        {
            $this->createTable('news', [
                'id' => $this->primaryKey(),
                'title' => $this->string()->notNull(),
                'content' => $this->text(),
            ]);

            $this->insert('news', [
                'title' => 'test 1',
                'content' => 'content 1',
            ]);
        }

        public function safeDown()
        {
            $this->delete('news', ['id' => 1]);
            $this->dropTable('news');
        }
    }

Note that usually when you perform multiple DB operations in `safeUp()`, you should reverse their execution order in `safeDown()`. In the above example we first create the table and then insert a row in `safeUp()`; while in `safeDown()` we first delete the row and then drop the table.

> Note: Not all DBMS support transactions. And some DB queries cannot be put into a transaction. For some examples, please refer to implicit commit. If this is the case, you should still implement `up()` and `down()`, instead.

### Database Accessing Methods ¶

The base migration class yii\db\Migration provides a set of methods to let you access and manipulate databases. You may find these methods are named similarly as the DAO methods provided by the yii\db\Command class. For example, the yii\db\Migration::createTable() method allows you to create a new table, just like yii\db\Command::createTable() does.

The benefit of using the methods provided by yii\db\Migration is that you do not need to explicitly create yii\db\Command instances and the execution of each method will automatically display useful messages telling you what database operations are done and how long they take.

Below is the list of all these database accessing methods:

- execute(): executing a SQL statement
- insert(): inserting a single row
- batchInsert(): inserting multiple rows
- update(): updating rows
- upsert(): inserting a single row or updating it if it exists (since 2.0.14)
- delete(): deleting rows
- createTable(): creating a table
- renameTable(): renaming a table
- dropTable(): removing a table
- truncateTable(): removing all rows in a table
- addColumn(): adding a column
- renameColumn(): renaming a column
- dropColumn(): removing a column
- alterColumn(): altering a column
- addPrimaryKey(): adding a primary key
- dropPrimaryKey(): removing a primary key
- addForeignKey(): adding a foreign key
- dropForeignKey(): removing a foreign key
- createIndex(): creating an index
- dropIndex(): removing an index
- addCommentOnColumn(): adding comment to column
- dropCommentFromColumn(): dropping comment from column
- addCommentOnTable(): adding comment to table
- dropCommentFromTable(): dropping comment from table

> Info: yii\db\Migration does not provide a database query method. This is because you normally do not need to display extra message about retrieving data from a database. It is also because you can use the powerful Query Builder to build and run complex queries. Using Query Builder in a migration may look like this:

>     // update status field for all users
>     foreach((new Query)->from('users')->each() as $user) {
>         $this->update('users', ['status' => 1], ['id' => $user['id']]);
>     }

## Applying Migrations ¶

To upgrade a database to its latest structure, you should apply all available new migrations using the following command:

    yii migrate

This command will list all migrations that have not been applied so far. If you confirm that you want to apply these migrations, it will run the `up()` or `safeUp()` method in every new migration class, one after another, in the order of their timestamp values. If any of the migrations fails, the command will quit without applying the rest of the migrations.

> Tip: In case you don't have command line at your server you may try web shell extension.

For each migration that has been successfully applied, the command will insert a row into a database table named `migration` to record the successful application of the migration. This will allow the migration tool to identify which migrations have been applied and which have not.

> Info: The migration tool will automatically create the `migration` table in the database specified by the db option of the command. By default, the database is specified by the `db` application component.

Sometimes, you may only want to apply one or a few new migrations, instead of all available migrations. You can do so by specifying the number of migrations that you want to apply when running the command. For example, the following command will try to apply the next three available migrations:

    yii migrate 3

You can also explicitly specify a particular migration to which the database should be migrated by using the `migrate/to` command in one of the following formats:

    yii migrate/to 150101_185401                      # using timestamp to specify the migration
    yii migrate/to "2015-01-01 18:54:01"              # using a string that can be parsed by strtotime()
    yii migrate/to m150101_185401_create_news_table   # using full name
    yii migrate/to 1392853618                         # using UNIX timestamp

If there are any unapplied migrations earlier than the specified one, they will all be applied before the specified migration is applied.

If the specified migration has already been applied before, any later applied migrations will be reverted.

## Reverting Migrations ¶

To revert (undo) one or multiple migrations that have been applied before, you can run the following command:

    yii migrate/down     # revert the most recently applied migration
    yii migrate/down 3   # revert the most 3 recently applied migrations

> Note: Not all migrations are reversible. Trying to revert such migrations will cause an error and stop the entire reverting process.

## Redoing Migrations ¶

Redoing migrations means first reverting the specified migrations and then applying again. This can be done as follows:

    yii migrate/redo        # redo the last applied migration
    yii migrate/redo 3      # redo the last 3 applied migrations

> Note: If a migration is not reversible, you will not be able to redo it.

## Refreshing Migrations ¶

Since Yii 2.0.13 you can delete all tables and foreign keys from the database and apply all migrations from the beginning.

    yii migrate/fresh       # truncate the database and apply all migrations from the beginning

## Listing Migrations ¶

To list which migrations have been applied and which are not, you may use the following commands:

    yii migrate/history     # showing the last 10 applied migrations
    yii migrate/history 5   # showing the last 5 applied migrations
    yii migrate/history all # showing all applied migrations

    yii migrate/new         # showing the first 10 new migrations
    yii migrate/new 5       # showing the first 5 new migrations
    yii migrate/new all     # showing all new migrations

## Modifying Migration History ¶

Instead of actually applying or reverting migrations, sometimes you may simply want to mark that your database has been upgraded to a particular migration. This often happens when you manually change the database to a particular state and you do not want the migration(s) for that change to be re-applied later. You can achieve this goal with the following command:

    yii migrate/mark 150101_185401                      # using timestamp to specify the migration
    yii migrate/mark "2015-01-01 18:54:01"              # using a string that can be parsed by strtotime()
    yii migrate/mark m150101_185401_create_news_table   # using full name
    yii migrate/mark 1392853618                         # using UNIX timestamp

The command will modify the `migration` table by adding or deleting certain rows to indicate that the database has been applied migrations to the specified one. No migrations will be applied or reverted by this command.

## Customizing Migrations ¶

There are several ways to customize the migration command.

### Using Command Line Options ¶

The migration command comes with a few command-line options that can be used to customize its behaviors:

- `interactive`: boolean (defaults to `true`), specifies whether to perform migrations in an interactive mode. When this is `true`, the user will be prompted before the command performs certain actions. You may want to set this to `false` if the command is being used in a background process.

- `migrationPath`: string|array (defaults to `@app/migrations`), specifies the directory storing all migration class files. This can be specified as either a directory path or a path alias. Note that the directory must exist, or the command may trigger an error. Since version 2.0.12 an array can be specified for loading migrations from multiple sources.

- `migrationTable`: string (defaults to `migration`), specifies the name of the database table for storing migration history information. The table will be automatically created by the command if it does not exist. You may also manually create it using the structure `version varchar(255) primary key, apply_time integer`.

- `db`: string (defaults to `db`), specifies the ID of the database application component. It represents the database that will be migrated using this command.

- `templateFile`: string (defaults to `@yii/views/migration.php`), specifies the path of the template file that is used for generating skeleton migration class files. This can be specified as either a file path or a path alias. The template file is a PHP script in which you can use a predefined variable named `$className` to get the migration class name.

- `generatorTemplateFiles`: array (defaults to `[

        'create_table' => '@yii/views/createTableMigration.php',
        'drop_table' => '@yii/views/dropTableMigration.php',
        'add_column' => '@yii/views/addColumnMigration.php',
        'drop_column' => '@yii/views/dropColumnMigration.php',
        'create_junction' => '@yii/views/createTableMigration.php'

  ]`), specifies template files for generating migration code. See "Generating Migrations" for more details.

- `fields`: array of column definition strings used for creating migration code. Defaults to `[]`. The format of each definition is `COLUMN_NAME:COLUMN_TYPE:COLUMN_DECORATOR`. For example, `--fields=name:string(12):notNull` produces a string column of size 12 which is not `null`.

The following example shows how you can use these options.

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:

    # migrate the migrations in a forum module non-interactively
    yii migrate --migrationPath=@app/modules/forum/migrations --interactive=0

### Configuring Command Globally ¶

Instead of entering the same option values every time you run the migration command, you may configure it once for all in the application configuration like shown below:

    return [
        'controllerMap' => [
            'migrate' => [
                'class' => 'yii\console\controllers\MigrateController',
                'migrationTable' => 'backend_migration',
            ],
        ],
    ];

With the above configuration, each time you run the migration command, the `backend_migration` table will be used to record the migration history. You no longer need to specify it via the `migrationTable` command-line option.

### Namespaced Migrations ¶

Since 2.0.10 you can use namespaces for the migration classes. You can specify the list of the migration namespaces via migrationNamespaces. Using of the namespaces for migration classes allows you usage of the several source locations for the migrations. For example:

    return [
        'controllerMap' => [
            'migrate' => [
                'class' => 'yii\console\controllers\MigrateController',
                'migrationPath' => null, // disable non-namespaced migrations if app\migrations is listed below
                'migrationNamespaces' => [
                    'app\migrations', // Common migrations for the whole application
                    'module\migrations', // Migrations for the specific project's module
                    'some\extension\migrations', // Migrations for the specific extension
                ],
            ],
        ],
    ];

> Note: Migrations applied from different namespaces will create a single migration history, e.g. you might be unable to apply or revert migrations from particular namespace only.

While operating namespaced migrations: creating new, reverting and so on, you should specify full namespace before migration name. Note that backslash (`\`) symbol is usually considered a special character in the shell, so you need to escape it properly to avoid shell errors or incorrect behavior. For example:

    yii migrate/create app\\migrations\\CreateUserTable

> Note: Migrations specified via migrationPath can not contain a namespace, namespaced migration can be applied only via yii\console\controllers\MigrateController::$migrationNamespaces property.

Since version 2.0.12 the migrationPath property also accepts an array for specifying multiple directories that contain migrations without a namespace. This is mainly added to be used in existing projects which use migrations from different locations. These migrations mainly come from external sources, like Yii extensions developed by other developers, which can not be changed to use namespaces easily when starting to use the new approach.

#### Generating namespaced migrations ¶

Namespaced migrations follow "CamelCase" naming pattern `M<YYMMDDHHMMSS><Name>` (for example `M190720100234CreateUserTable`). When generating such migration remember that table name will be converted from "CamelCase" format to "underscored". For example:

    yii migrate/create app\\migrations\\DropGreenHotelTable

generates migration within namespace `app\migrations` dropping table `green_hotel` and

    yii migrate/create app\\migrations\\CreateBANANATable

generates migration within namespace `app\migrations` creating table `b_a_n_a_n_a`.

If table's name is mixed-cased (like `studentsExam`) you need to precede the name with underscore:

    yii migrate/create app\\migrations\\Create_studentsExamTable

This generates migration within namespace `app\migrations` creating table `studentsExam`.

### Separated Migrations ¶

Sometimes using single migration history for all project migrations is not desirable. For example: you may install some 'blog' extension, which contains fully separated functionality and contain its own migrations, which should not affect the ones dedicated to main project functionality.

If you want several migrations to be applied and tracked down completely separated from each other, you can configure multiple migration commands which will use different namespaces and migration history tables:

    return [
        'controllerMap' => [
            // Common migrations for the whole application
            'migrate-app' => [
                'class' => 'yii\console\controllers\MigrateController',
                'migrationNamespaces' => ['app\migrations'],
                'migrationTable' => 'migration_app',
                'migrationPath' => null,
            ],
            // Migrations for the specific project's module
            'migrate-module' => [
                'class' => 'yii\console\controllers\MigrateController',
                'migrationNamespaces' => ['module\migrations'],
                'migrationTable' => 'migration_module',
                'migrationPath' => null,
            ],
            // Migrations for the specific extension
            'migrate-rbac' => [
                'class' => 'yii\console\controllers\MigrateController',
                'migrationPath' => '@yii/rbac/migrations',
                'migrationTable' => 'migration_rbac',
            ],
        ],
    ];

Note that to synchronize database you now need to run multiple commands instead of one:

    yii migrate-app
    yii migrate-module
    yii migrate-rbac

## Migrating Multiple Databases ¶

By default, migrations are applied to the same database specified by the `db` application component. If you want them to be applied to a different database, you may specify the `db` command-line option like shown below,

    yii migrate --db=db2

The above command will apply migrations to the `db2` database.

Sometimes it may happen that you want to apply some of the migrations to one database, while some others to another database. To achieve this goal, when implementing a migration class you should explicitly specify the DB component ID that the migration would use, like the following:

    <?php

    use yii\db\Migration;

    class m150101_185401_create_news_table extends Migration
    {
        public function init()
        {
            $this->db = 'db2';
            parent::init();
        }
    }

The above migration will be applied to `db2`, even if you specify a different database through the `db` command-line option. Note that the migration history will still be recorded in the database specified by the `db` command-line option.

If you have multiple migrations that use the same database, it is recommended that you create a base migration class with the above `init()` code. Then each migration class can extend from this base class.

> Tip: Besides setting the db property, you can also operate on different databases by creating new database connections to them in your migration classes. You then use the DAO methods with these connections to manipulate different databases.

Another strategy that you can take to migrate multiple databases is to keep migrations for different databases in different migration paths. Then you can migrate these databases in separate commands like the following:

    yii migrate --migrationPath=@app/migrations/db1 --db=db1
    yii migrate --migrationPath=@app/migrations/db2 --db=db2
    ...

The first command will apply migrations in `@app/migrations/db1` to the `db1` database, the second command will apply migrations in `@app/migrations/db2` to `db2`, and so on.