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 upand downthat 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 nameand 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_nameto new_name.

  • add_column(table_name, column_name, type, options): Adds a new column to the table called table_namenamed column_namespecified 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 optionshash like { :default => 11 }. Other options include :limitand :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_namefrom 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 :nameand :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::IrreversibleMigrationexception in their downmethod.

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.rbin the db/migrate/directory where nnnis the next largest migration number.

You may then edit the self.upand 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_migrationstable (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=Xwhere Xis the version to which you wish to downgrade. If any of the migrations throw an ActiveRecord::IrreversibleMigrationexception, 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_timemethod:

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_infowhen using migrations. This table contained the version of the schema as of the last applied migration.

Starting with Rails 2.1, the schema_infotable is (automatically) replaced by the schema_migrationstable, 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
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Class Public methods
announce(message)
# File activerecord/lib/active_record/migration.rb, line 315
def announce(message)
  text = "#{@version} #{name}: #{message}"
  length = [0, 75 - text.length].max
  write "== %s %s" % [text, "=" * length]
end
connection()
# File activerecord/lib/active_record/migration.rb, line 341
def connection
  ActiveRecord::Base.connection
end
method_missing(method, *arguments, &block)
# File activerecord/lib/active_record/migration.rb, line 345
def method_missing(method, *arguments, &block)
  arg_list = arguments.map(&:inspect) * ', '
  say_with_time "#{method}(#{arg_list})" do
    unless arguments.empty? || method == :execute
      arguments[0] = Migrator.proper_table_name(arguments.first)
    end
    connection.send(method, *arguments, &block)
  end
end
migrate(direction)

Execute this migration in the named direction

# File activerecord/lib/active_record/migration.rb, line 273
def migrate(direction)
  return unless respond_to?(direction)
  case direction
    when :up   then announce "migrating"
    when :down then announce "reverting"
  end
  result = nil
  time = Benchmark.measure { result = send("#{direction}_without_benchmarks") }
  case direction
    when :up   then announce "migrated (%.4fs)" % time.real; write
    when :down then announce "reverted (%.4fs)" % time.real; write
  end
  result
end
say(message, subitem=false)
# File activerecord/lib/active_record/migration.rb, line 321
def say(message, subitem=false)
  write "#{subitem ? "   ->" : "--"} #{message}"
end
say_with_time(message)
# File activerecord/lib/active_record/migration.rb, line 325
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()
# File activerecord/lib/active_record/migration.rb, line 334
def suppress_messages
  save, self.verbose = verbose, false
  yield
ensure
  self.verbose = save
end
write(text="")
# File activerecord/lib/active_record/migration.rb, line 311
def write(text="")
  puts(text) if verbose
end