Class: ActiveRecord::Migration

### Home Classes Methods

### In Files

- lib/active_record/migration.rb
- lib/active_record/migration/command_recorder.rb

### Parent

Object

### Namespace

- CLASS ActiveRecord::Migration::CommandRecorder

### Methods

- ::migrate
- ::new
- #announce
- #connection
- #copy
- #down
- #method_missing
- #migrate
- #next_migration_number
- #revert
- #reverting?
- #say
- #say_with_time
- #suppress_messages
- #up
- #write

### Files

- README.rdoc

### Class/Module Index

Quicksearch

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No matching classes.

# ActiveRecord::Migration

# Active Record Migrations

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 up
        add_column :accounts, :ssl_enabled, :boolean, :default => 1
      end

      def 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 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 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 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_names)`: Removes the column listed in `column_names` 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`, `:unique` (e.g. `{ :name => "users_name_index", :unique => true }`) and `:order` (e.g. { :order => {:name => :desc} }</tt>).

- `remove_index(table_name, :column => column_name)`: Removes the index specified by `column_name`.

- `remove_index(table_name, :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

    rails generate migration MyNewMigration

where MyNewMigration is the name of your migration. The generator will create an empty migration file `timestamp_my_new_migration.rb` in the `db/migrate/` directory where `timestamp` is the UTC formatted date and time that the migration was generated.

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

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

    rails generate migration add_fieldname_to_tablename fieldname:string

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

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

      def 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 up
        Tag.all.each { |tag| tag.destroy if tag.pages.empty? }
      end

      def 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 up
        remove_column :items, :incomplete_items_count
        remove_column :items, :completed_items_count
      end

      def 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 up
        execute "ALTER TABLE `pages_linked_pages` ADD UNIQUE `page_id_linked_page_id` (`page_id`,`linked_page_id`)"
      end

      def 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 up
        add_column :people, :salary, :integer
        Person.reset_column_information
        Person.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 up
      ...
      say_with_time "Updating salaries..." do
        Person.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 application.rb.

## Reversible Migrations

Starting with Rails 3.1, you will be able to define reversible migrations. Reversible migrations are migrations that know how to go `down` for you. You simply supply the `up` logic, and the Migration system will figure out how to execute the down commands for you.

To define a reversible migration, define the `change` method in your migration like this:

    class TenderloveMigration < ActiveRecord::Migration
      def change
        create_table(:horses) do |t|
          t.column :content, :text
          t.column :remind_at, :datetime
        end
      end
    end

This migration will create the horses table for you on the way up, and automatically figure out how to drop the table on the way down.

Some commands like `remove_column` cannot be reversed. If you care to define how to move up and down in these cases, you should define the `up` and `down` methods as before.

If a command cannot be reversed, an `ActiveRecord::IrreversibleMigration` exception will be raised when the migration is moving down.

For a list of commands that are reversible, please see `ActiveRecord::Migration::CommandRecorder`.

### Attributes

name[RW]
version[RW]

### Public Class Methods

migrate(direction) click to toggle source

    # File lib/active_record/migration.rb, line 337
    def self.migrate(direction)
      new.migrate direction
    end

new() click to toggle source

    # File lib/active_record/migration.rb, line 345
    def initialize
      @name       = self.class.name
      @version    = nil
      @connection = nil
      @reverting  = false
    end

### Public Instance Methods

announce(message) click to toggle source

    # File lib/active_record/migration.rb, line 425
    def announce(message)
      text = "#{version} #{name}: #{message}"
      length = [0, 75 - text.length].max
      write "== %s %s" % [text, "=" * length]
    end

connection() click to toggle source

    # File lib/active_record/migration.rb, line 451
    def connection
      @connection || ActiveRecord::Base.connection
    end

copy(destination, sources, options = {}) click to toggle source

    # File lib/active_record/migration.rb, line 470
    def copy(destination, sources, options = {})
      copied = []

      FileUtils.mkdir_p(destination) unless File.exists?(destination)

      destination_migrations = ActiveRecord::Migrator.migrations(destination)
      last = destination_migrations.last
      sources.each do |scope, path|
        source_migrations = ActiveRecord::Migrator.migrations(path)

        source_migrations.each do |migration|
          source = File.read(migration.filename)
          source = "# This migration comes from #{scope} (originally #{migration.version})\n#{source}"

          if duplicate = destination_migrations.detect { |m| m.name == migration.name }
            if options[:on_skip] && duplicate.scope != scope.to_s
              options[:on_skip].call(scope, migration)
            end
            next
          end

          migration.version = next_migration_number(last ? last.version + 1 : 0).to_i
          new_path = File.join(destination, "#{migration.version}_#{migration.name.underscore}.#{scope}.rb")
          old_path, migration.filename = migration.filename, new_path
          last = migration

          File.open(migration.filename, "w") { |f| f.write source }
          copied << migration
          options[:on_copy].call(scope, migration, old_path) if options[:on_copy]
          destination_migrations << migration
        end
      end

      copied
    end

down() click to toggle source

    # File lib/active_record/migration.rb, line 373
    def down
      self.class.delegate = self
      return unless self.class.respond_to?(:down)
      self.class.down
    end

method_missing(method, *arguments, &block) click to toggle source

    # File lib/active_record/migration.rb, line 455
    def method_missing(method, *arguments, &block)
      arg_list = arguments.map{ |a| a.inspect } * ', '

      say_with_time "#{method}(#{arg_list})" do
        unless reverting?
          unless arguments.empty? || method == :execute
            arguments[0] = Migrator.proper_table_name(arguments.first) unless method == :assume_migrated_upto_version
            arguments[1] = Migrator.proper_table_name(arguments.second) if method == :rename_table
          end
        end
        return super unless connection.respond_to?(method)
        connection.send(method, *arguments, &block)
      end
    end

migrate(direction) click to toggle source

Execute this migration in the named direction

    # File lib/active_record/migration.rb, line 380
    def migrate(direction)
      return unless respond_to?(direction)

      case direction
      when :up   then announce "migrating"
      when :down then announce "reverting"
      end

      time   = nil
      ActiveRecord::Base.connection_pool.with_connection do |conn|
        @connection = conn
        if respond_to?(:change)
          if direction == :down
            recorder = CommandRecorder.new(@connection)
            suppress_messages do
              @connection = recorder
              change
            end
            @connection = conn
            time = Benchmark.measure {
              self.revert {
                recorder.inverse.each do |cmd, args|
                  send(cmd, *args)
                end
              }
            }
          else
            time = Benchmark.measure { change }
          end
        else
          time = Benchmark.measure { send(direction) }
        end
        @connection = nil
      end

      case direction
      when :up   then announce "migrated (%.4fs)" % time.real; write
      when :down then announce "reverted (%.4fs)" % time.real; write
      end
    end

next_migration_number(number) click to toggle source

    # File lib/active_record/migration.rb, line 506
    def next_migration_number(number)
      if ActiveRecord::Base.timestamped_migrations
        [Time.now.utc.strftime("%Y%m%d%H%M%S"), "%.14d" % number].max
      else
        "%.3d" % number
      end
    end

revert() click to toggle source

    # File lib/active_record/migration.rb, line 356
    def revert
      @reverting = true
      yield
    ensure
      @reverting = false
    end

reverting?() click to toggle source

    # File lib/active_record/migration.rb, line 363
    def reverting?
      @reverting
    end

say(message, subitem=false) click to toggle source

    # File lib/active_record/migration.rb, line 431
    def say(message, subitem=false)
      write "#{subitem ? "   ->" : "--"} #{message}"
    end

say_with_time(message) click to toggle source

    # File lib/active_record/migration.rb, line 435
    def say_with_time(message)
      say(message)
      result = nil
      time = Benchmark.measure { result = yield }
      say "%.4fs" % time.real, :subitem
      say("#{result} rows", :subitem) if result.is_a?(Integer)
      result
    end

suppress_messages() click to toggle source

    # File lib/active_record/migration.rb, line 444
    def suppress_messages
      save, self.verbose = verbose, false
      yield
    ensure
      self.verbose = save
    end

up() click to toggle source

    # File lib/active_record/migration.rb, line 367
    def up
      self.class.delegate = self
      return unless self.class.respond_to?(:up)
      self.class.up
    end

write(text="") click to toggle source

    # File lib/active_record/migration.rb, line 421
    def write(text="")
      puts(text) if verbose
    end

[Validate]

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