Class: ActiveRecord::Migration

Class | ActiveRecord::Migration
In: | lib/active_record/migration.rb
Parent: | Object

Migrations can manage the evolution of a schema used by several physical databases. It‘s a solution to the common problem of adding a field to make a new feature work in your local database, but being unsure of how to push that change to other developers and to the production server. With migrations, you can describe the transformations in self-contained classes that can be checked into version control systems and executed against another database that might be one, two, or five versions behind.

Example of a simple migration:

      class AddSsl < ActiveRecord::Migration
        def self.up
          add_column :accounts, :ssl_enabled, :boolean, :default => 1
        end

        def self.down
          remove_column :accounts, :ssl_enabled
        end
      end

This migration will add a boolean flag to the accounts table and remove it if you‘re backing out of the migration. It shows how all migrations have two class methods `up` and `down` that describes the transformations required to implement or remove the migration. These methods can consist of both the migration specific methods like add_column and remove_column, but may also contain regular Ruby code for generating data needed for the transformations.

Example of a more complex migration that also needs to initialize data:

      class AddSystemSettings < ActiveRecord::Migration
        def self.up
          create_table :system_settings do |t|
            t.string  :name
            t.string  :label
            t.text  :value
            t.string  :type
            t.integer  :position
          end

          SystemSetting.create :name => "notice", :label => "Use notice?", :value => 1
        end

        def self.down
          drop_table :system_settings
        end
      end

This migration first adds the system_settings table, then creates the very first row in it using the Active Record model that relies on the table. It also uses the more advanced create_table syntax where you can specify a complete table schema in one block call.

## Available transformations

- `create_table(name, options)` Creates a table called `name` and makes the table object available to a block that can then add columns to it, following the same format as add_column. See example above. The options hash is for fragments like "DEFAULT CHARSET=UTF-8" that are appended to the create table definition.
- `drop_table(name)`: Drops the table called `name`.
- `rename_table(old_name, new_name)`: Renames the table called `old_name` to `new_name`.
- `add_column(table_name, column_name, type, options)`: Adds a new column to the table called `table_name` named `column_name` specified to be one of the following types: `:string`, `:text`, `:integer`, `:float`, `:decimal`, `:datetime`, `:timestamp`, `:time`, `:date`, `:binary`, `:boolean`. A default value can be specified by passing an `options` hash like `{ :default => 11 }`. Other options include `:limit` and `:null` (e.g. `{ :limit => 50, :null => false }`) — see ActiveRecord::ConnectionAdapters::TableDefinition#column for details.
- `rename_column(table_name, column_name, new_column_name)`: Renames a column but keeps the type and content.
- `change_column(table_name, column_name, type, options)`: Changes the column to a different type using the same parameters as add_column.
- `remove_column(table_name, column_name)`: Removes the column named `column_name` from the table called `table_name`.
- `add_index(table_name, column_names, options)`: Adds a new index with the name of the column. Other options include `:name` and `:unique` (e.g. `{ :name => "users_name_index", :unique => true }`).
- `remove_index(table_name, index_name)`: Removes the index specified by `index_name`.

## Irreversible transformations

Some transformations are destructive in a manner that cannot be reversed. Migrations of that kind should raise an `ActiveRecord::IrreversibleMigration` exception in their `down` method.

## Running migrations from within Rails

The Rails package has several tools to help create and apply migrations.

To generate a new migration, you can use

      script/generate migration MyNewMigration

where MyNewMigration is the name of your migration. The generator will create an empty migration file `nnn_my_new_migration.rb` in the `db/migrate/` directory where `nnn` is the next largest migration number.

You may then edit the `self.up` and `self.down` methods of MyNewMigration.

There is a special syntactic shortcut to generate migrations that add fields to a table.

      script/generate migration add_fieldname_to_tablename fieldname:string

This will generate the file `nnn_add_fieldname_to_tablename`, which will look like this:

      class AddFieldnameToTablename < ActiveRecord::Migration
        def self.up
          add_column :tablenames, :fieldname, :string
        end

        def self.down
          remove_column :tablenames, :fieldname
        end
      end

To run migrations against the currently configured database, use `rake db:migrate`. This will update the database by running all of the pending migrations, creating the `schema_migrations` table (see "About the schema_migrations table" section below) if missing. It will also invoke the db:schema:dump task, which will update your db/schema.rb file to match the structure of your database.

To roll the database back to a previous migration version, use `rake db:migrate VERSION=X` where `X` is the version to which you wish to downgrade. If any of the migrations throw an `ActiveRecord::IrreversibleMigration` exception, that step will fail and you‘ll have some manual work to do.

## Database support

Migrations are currently supported in MySQL, PostgreSQL, SQLite, SQL Server, Sybase, and Oracle (all supported databases except DB2).

## More examples

Not all migrations change the schema. Some just fix the data:

      class RemoveEmptyTags < ActiveRecord::Migration
        def self.up
          Tag.find(:all).each { |tag| tag.destroy if tag.pages.empty? }
        end

        def self.down
          # not much we can do to restore deleted data
          raise ActiveRecord::IrreversibleMigration, "Can't recover the deleted tags"
        end
      end

Others remove columns when they migrate up instead of down:

      class RemoveUnnecessaryItemAttributes < ActiveRecord::Migration
        def self.up
          remove_column :items, :incomplete_items_count
          remove_column :items, :completed_items_count
        end

        def self.down
          add_column :items, :incomplete_items_count
          add_column :items, :completed_items_count
        end
      end

And sometimes you need to do something in SQL not abstracted directly by migrations:

      class MakeJoinUnique < ActiveRecord::Migration
        def self.up
          execute "ALTER TABLE `pages_linked_pages` ADD UNIQUE `page_id_linked_page_id` (`page_id`,`linked_page_id`)"
        end

        def self.down
          execute "ALTER TABLE `pages_linked_pages` DROP INDEX `page_id_linked_page_id`"
        end
      end

## Using a model after changing its table

Sometimes you‘ll want to add a column in a migration and populate it immediately after. In that case, you‘ll need to make a call to Base#reset_column_information in order to ensure that the model has the latest column data from after the new column was added. Example:

      class AddPeopleSalary < ActiveRecord::Migration
        def self.up
          add_column :people, :salary, :integer
          Person.reset_column_information
          Person.find(:all).each do |p|
            p.update_attribute :salary, SalaryCalculator.compute(p)
          end
        end
      end

## Controlling verbosity

By default, migrations will describe the actions they are taking, writing them to the console as they happen, along with benchmarks describing how long each step took.

You can quiet them down by setting ActiveRecord::Migration.verbose = false.

You can also insert your own messages and benchmarks by using the `say_with_time` method:

      def self.up
        ...
        say_with_time "Updating salaries..." do
          Person.find(:all).each do |p|
            p.update_attribute :salary, SalaryCalculator.compute(p)
          end
        end
        ...
      end

The phrase "Updating salaries…" would then be printed, along with the benchmark for the block when the block completes.

## About the schema_migrations table

Rails versions 2.0 and prior used to create a table called `schema_info` when using migrations. This table contained the version of the schema as of the last applied migration.

Starting with Rails 2.1, the `schema_info` table is (automatically) replaced by the `schema_migrations` table, which contains the version numbers of all the migrations applied.

As a result, it is now possible to add migration files that are numbered lower than the current schema version: when migrating up, those never-applied "interleaved" migrations will be automatically applied, and when migrating down, never-applied "interleaved" migrations will be skipped.

## Timestamped Migrations

By default, Rails generates migrations that look like:

       20080717013526_your_migration_name.rb

The prefix is a generation timestamp (in UTC).

If you‘d prefer to use numeric prefixes, you can turn timestamped migrations off by setting:

       config.active_record.timestamped_migrations = false

In environment.rb.

### Methods

announce connection method_missing migrate say say_with_time suppress_messages write

### Public Class methods

announce(message)
connection()
method_missing(method, *arguments, &block)
migrate(direction)

Execute this migration in the named direction

say(message, subitem=false)
say_with_time(message) {|| ...}
suppress_messages() {|| ...}
write(text="")

[Validate]