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 again, 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.column :name, :string t.column :label, :string t.column :value, :text t.column :type, :string t.column :position, :integer 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.
Some transformations are destructive in a manner that cannot be reversed. Migrations of that kind should raise an IrreversibleMigration exception in their down method.
The Rails package has support for migrations with the script/generate migration my_new_migration command and with the rake migrate command that’ll run all the pending migrations. It‘ll even create the needed schema_info table automatically if it’s missing.
Migrations are currently only supported in MySQL and PostgreSQL.
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 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 some times 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