path: root/activerecord/lib/active_record/migration.rb

require "active_support/core_ext/class/attribute_accessors"
require 'set'

module ActiveRecord
  # Exception that can be raised to stop migrations from going backwards.
  class IrreversibleMigration < ActiveRecordError
  end

  class DuplicateMigrationVersionError < ActiveRecordError#:nodoc:
    def initialize(version)
      super("Multiple migrations have the version number #{version}")
    end
  end

  class DuplicateMigrationNameError < ActiveRecordError#:nodoc:
    def initialize(name)
      super("Multiple migrations have the name #{name}")
    end
  end

  class UnknownMigrationVersionError < ActiveRecordError #:nodoc:
    def initialize(version)
      super("No migration with version number #{version}")
    end
  end

  class IllegalMigrationNameError < ActiveRecordError#:nodoc:
    def initialize(name)
      super("Illegal name for migration file: #{name}\n\t(only lower case letters, numbers, and '_' allowed)")
    end
  end

  class PendingMigrationError < ActiveRecordError#:nodoc:
    def initialize
      super("Migrations are pending; run 'rake db:migrate RAILS_ENV=#{Rails.env}' to resolve this issue.")
    end
  end

  # = 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: true
  #     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
  #
  # * <tt>create_table(name, options)</tt>: 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.
  # * <tt>drop_table(name)</tt>: Drops the table called +name+.
  # * <tt>change_table(name, options)</tt>: Allows to make column alterations to
  #   the table called +name+. It makes the table object available to a block that
  #   can then add/remove columns, indexes or foreign keys to it.
  # * <tt>rename_table(old_name, new_name)</tt>: Renames the table called +old_name+
  #   to +new_name+.
  # * <tt>add_column(table_name, column_name, type, options)</tt>: Adds a new column
  #   to the table called +table_name+
  #   named +column_name+ specified to be one of the following types:
  #   <tt>:string</tt>, <tt>:text</tt>, <tt>:integer</tt>, <tt>:float</tt>,
  #   <tt>:decimal</tt>, <tt>:datetime</tt>, <tt>:timestamp</tt>, <tt>:time</tt>,
  #   <tt>:date</tt>, <tt>:binary</tt>, <tt>:boolean</tt>. A default value can be
  #   specified by passing an +options+ hash like <tt>{ default: 11 }</tt>.
  #   Other options include <tt>:limit</tt> and <tt>:null</tt> (e.g.
  #   <tt>{ limit: 50, null: false }</tt>) -- see
  #   ActiveRecord::ConnectionAdapters::TableDefinition#column for details.
  # * <tt>rename_column(table_name, column_name, new_column_name)</tt>: Renames
  #   a column but keeps the type and content.
  # * <tt>change_column(table_name, column_name, type, options)</tt>:  Changes
  #   the column to a different type using the same parameters as add_column.
  # * <tt>remove_column(table_name, column_names)</tt>: Removes the column listed in
  #   +column_names+ from the table called +table_name+.
  # * <tt>add_index(table_name, column_names, options)</tt>: Adds a new index
  #   with the name of the column. Other options include
  #   <tt>:name</tt>, <tt>:unique</tt> (e.g.
  #   <tt>{ name: 'users_name_index', unique: true }</tt>) and <tt>:order</tt>
  #   (e.g. <tt>{ order: { name: :desc } }</tt>).
  # * <tt>remove_index(table_name, column: column_name)</tt>: Removes the index
  #   specified by +column_name+.
  # * <tt>remove_index(table_name, name: index_name)</tt>: 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 <tt>ActiveRecord::IrreversibleMigration</tt>
  # 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 <tt>timestamp_my_new_migration.rb</tt>
  # in the <tt>db/migrate/</tt> directory where <tt>timestamp</tt> is the
  # UTC formatted date and time that the migration was generated.
  #
  # You may then edit the <tt>up</tt> and <tt>down</tt> 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 <tt>timestamp_add_fieldname_to_tablename</tt>, 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
  # <tt>rake db:migrate</tt>. This will update the database by running all of the
  # pending migrations, creating the <tt>schema_migrations</tt> 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
  # <tt>rake db:migrate VERSION=X</tt> where <tt>X</tt> is the version to which
  # you wish to downgrade. If any of the migrations throw an
  # <tt>ActiveRecord::IrreversibleMigration</tt> 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
  # <tt>Base#reset_column_information</tt> 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
  # <tt>schema_info</tt> when using migrations. This table contained the
  # version of the schema as of the last applied migration.
  #
  # Starting with Rails 2.1, the <tt>schema_info</tt> table is
  # (automatically) replaced by the <tt>schema_migrations</tt> 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
  # <tt>ActiveRecord::IrreversibleMigration</tt> exception will be raised when
  # the migration is moving down.
  #
  # For a list of commands that are reversible, please see
  # <tt>ActiveRecord::Migration::CommandRecorder</tt>.
  #
  # == Transactional Migrations
  #
  # If the database adapter supports DDL transactions, all migrations will
  # automatically be wrapped in a transaction. There are queries that you
  # can't execute inside a transaction though, and for these situations
  # you can turn the automatic transactions off.
  #
  #   class ChangeEnum < ActiveRecord::Migration
  #     disable_ddl_transaction!
  #
  #     def up
  #       execute "ALTER TYPE model_size ADD VALUE 'new_value'"
  #     end
  #   end
  #
  # Remember that you can still open your own transactions, even if you
  # are in a Migration with <tt>self.disable_ddl_transaction!</tt>.
  class Migration
    autoload :CommandRecorder, 'active_record/migration/command_recorder'


    # This class is used to verify that all migrations have been run before
    # loading a web page if config.active_record.migration_error is set to :page_load
    class CheckPending
      def initialize(app)
        @app = app
      end

      def call(env)
        ActiveRecord::Base.logger.silence do
          ActiveRecord::Migration.check_pending!
        end
        @app.call(env)
      end
    end

    class << self
      attr_accessor :delegate # :nodoc:
      attr_accessor :disable_ddl_transaction # :nodoc:
    end

    def self.check_pending!
      raise ActiveRecord::PendingMigrationError if ActiveRecord::Migrator.needs_migration?
    end

    def self.method_missing(name, *args, &block) # :nodoc:
      (delegate || superclass.delegate).send(name, *args, &block)
    end

    def self.migrate(direction)
      new.migrate direction
    end

    # Disable DDL transactions for this migration.
    def self.disable_ddl_transaction!
      @disable_ddl_transaction = true
    end

    def disable_ddl_transaction # :nodoc:
      self.class.disable_ddl_transaction
    end

    cattr_accessor :verbose
    attr_accessor :name, :version

    def initialize(name = self.class.name, version = nil)
      @name       = name
      @version    = version
      @connection = nil
    end

    self.verbose = true
    # instantiate the delegate object after initialize is defined
    self.delegate = new

    # Reverses the migration commands for the given block and
    # the given migrations.
    #
    # The following migration will remove the table 'horses'
    # and create the table 'apples' on the way up, and the reverse
    # on the way down.
    #
    #   class FixTLMigration < ActiveRecord::Migration
    #     def change
    #       revert do
    #         create_table(:horses) do |t|
    #           t.text :content
    #           t.datetime :remind_at
    #         end
    #       end
    #       create_table(:apples) do |t|
    #         t.string :variety
    #       end
    #     end
    #   end
    #
    # Or equivalently, if +TenderloveMigration+ is defined as in the
    # documentation for Migration:
    #
    #   require_relative '2012121212_tenderlove_migration'
    #
    #   class FixupTLMigration < ActiveRecord::Migration
    #     def change
    #       revert TenderloveMigration
    #
    #       create_table(:apples) do |t|
    #         t.string :variety
    #       end
    #     end
    #   end
    #
    # This command can be nested.
    def revert(*migration_classes)
      run(*migration_classes.reverse, revert: true) unless migration_classes.empty?
      if block_given?
        if @connection.respond_to? :revert
          @connection.revert { yield }
        else
          recorder = CommandRecorder.new(@connection)
          @connection = recorder
          suppress_messages do
            @connection.revert { yield }
          end
          @connection = recorder.delegate
          recorder.commands.each do |cmd, args, block|
            send(cmd, *args, &block)
          end
        end
      end
    end

    def reverting?
      @connection.respond_to?(:reverting) && @connection.reverting
    end

    class ReversibleBlockHelper < Struct.new(:reverting)
      def up
        yield unless reverting
      end

      def down
        yield if reverting
      end
    end

    # Used to specify an operation that can be run in one direction or another.
    # Call the methods +up+ and +down+ of the yielded object to run a block
    # only in one given direction.
    # The whole block will be called in the right order within the migration.
    #
    # In the following example, the looping on users will always be done
    # when the three columns 'first_name', 'last_name' and 'full_name' exist,
    # even when migrating down:
    #
    #    class SplitNameMigration < ActiveRecord::Migration
    #      def change
    #        add_column :users, :first_name, :string
    #        add_column :users, :last_name, :string
    #
    #        reversible do |dir|
    #          User.reset_column_information
    #          User.all.each do |u|
    #            dir.up   { u.first_name, u.last_name = u.full_name.split(' ') }
    #            dir.down { u.full_name = "#{u.first_name} #{u.last_name}" }
    #            u.save
    #          end
    #        end
    #
    #        revert { add_column :users, :full_name, :string }
    #      end
    #    end
    def reversible
      helper = ReversibleBlockHelper.new(reverting?)
      execute_block{ yield helper }
    end

    # Runs the given migration classes.
    # Last argument can specify options:
    # - :direction (default is :up)
    # - :revert (default is false)
    def run(*migration_classes)
      opts = migration_classes.extract_options!
      dir = opts[:direction] || :up
      dir = (dir == :down ? :up : :down) if opts[:revert]
      if reverting?
        # If in revert and going :up, say, we want to execute :down without reverting, so
        revert { run(*migration_classes, direction: dir, revert: true) }
      else
        migration_classes.each do |migration_class|
          migration_class.new.exec_migration(@connection, dir)
        end
      end
    end

    def up
      self.class.delegate = self
      return unless self.class.respond_to?(:up)
      self.class.up
    end

    def down
      self.class.delegate = self
      return unless self.class.respond_to?(:down)
      self.class.down
    end

    # Execute this migration in the named direction
    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|
        time = Benchmark.measure do
          exec_migration(conn, direction)
        end
      end

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

    def exec_migration(conn, direction)
      @connection = conn
      if respond_to?(:change)
        if direction == :down
          revert { change }
        else
          change
        end
      else
        send(direction)
      end
    ensure
      @connection = nil
    end

    def write(text="")
      puts(text) if verbose
    end

    def announce(message)
      text = "#{version} #{name}: #{message}"
      length = [0, 75 - text.length].max
      write "== %s %s" % [text, "=" * length]
    end

    def say(message, subitem=false)
      write "#{subitem ? "   ->" : "--"} #{message}"
    end

    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

    def suppress_messages
      save, self.verbose = verbose, false
      yield
    ensure
      self.verbose = save
    end

    def connection
      @connection || ActiveRecord::Base.connection
    end

    def method_missing(method, *arguments, &block)
      arg_list = arguments.map{ |a| a.inspect } * ', '

      say_with_time "#{method}(#{arg_list})" do
        unless @connection.respond_to? :revert
          unless arguments.empty? || method == :execute
            arguments[0] = Migrator.proper_table_name(arguments.first)
            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

    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.binread(migration.filename)
          inserted_comment = "# This migration comes from #{scope} (originally #{migration.version})\n"
          if /\A#.*\b(?:en)?coding:\s*\S+/ =~ source
            # If we have a magic comment in the original migration,
            # insert our comment after the first newline(end of the magic comment line)
            # so the magic keep working.
            # Note that magic comments must be at the first line(except sh-bang).
            source[/\n/] = "\n#{inserted_comment}"
          else
            source = "#{inserted_comment}#{source}"
          end

          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.binwrite(migration.filename, source)
          copied << migration
          options[:on_copy].call(scope, migration, old_path) if options[:on_copy]
          destination_migrations << migration
        end
      end

      copied
    end

    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

    private
    def execute_block
      if connection.respond_to? :execute_block
        super # use normal delegation to record the block
      else
        yield
      end
    end
  end

  # MigrationProxy is used to defer loading of the actual migration classes
  # until they are needed
  class MigrationProxy < Struct.new(:name, :version, :filename, :scope)

    def initialize(name, version, filename, scope)
      super
      @migration = nil
    end

    def basename
      File.basename(filename)
    end

    delegate :migrate, :announce, :write, :disable_ddl_transaction, to: :migration

    private

      def migration
        @migration ||= load_migration
      end

      def load_migration
        require(File.expand_path(filename))
        name.constantize.new
      end

  end

  class Migrator#:nodoc:
    class << self
      attr_writer :migrations_paths
      alias :migrations_path= :migrations_paths=

      def migrate(migrations_paths, target_version = nil, &block)
        case
        when target_version.nil?
          up(migrations_paths, target_version, &block)
        when current_version == 0 && target_version == 0
          []
        when current_version > target_version
          down(migrations_paths, target_version, &block)
        else
          up(migrations_paths, target_version, &block)
        end
      end

      def rollback(migrations_paths, steps=1)
        move(:down, migrations_paths, steps)
      end

      def forward(migrations_paths, steps=1)
        move(:up, migrations_paths, steps)
      end

      def up(migrations_paths, target_version = nil)
        migrations = migrations(migrations_paths)
        migrations.select! { |m| yield m } if block_given?

        self.new(:up, migrations, target_version).migrate
      end

      def down(migrations_paths, target_version = nil, &block)
        migrations = migrations(migrations_paths)
        migrations.select! { |m| yield m } if block_given?

        self.new(:down, migrations, target_version).migrate
      end

      def run(direction, migrations_paths, target_version)
        self.new(direction, migrations(migrations_paths), target_version).run
      end

      def open(migrations_paths)
        self.new(:up, migrations(migrations_paths), nil)
      end

      def schema_migrations_table_name
        SchemaMigration.table_name
      end

      def get_all_versions
        SchemaMigration.all.map { |x| x.version.to_i }.sort
      end

      def current_version
        sm_table = schema_migrations_table_name
        if Base.connection.table_exists?(sm_table)
          get_all_versions.max || 0
        else
          0
        end
      end

      def needs_migration?
        current_version < last_version
      end

      def last_version
        migrations(migrations_paths).last.try(:version)||0
      end

      def proper_table_name(name)
        # Use the Active Record objects own table_name, or pre/suffix from ActiveRecord::Base if name is a symbol/string
        if name.respond_to? :table_name
          name.table_name
        else
          "#{ActiveRecord::Base.table_name_prefix}#{name}#{ActiveRecord::Base.table_name_suffix}"
        end
      end

      def migrations_paths
        @migrations_paths ||= ['db/migrate']
        # just to not break things if someone uses: migration_path = some_string
        Array(@migrations_paths)
      end

      def migrations_path
        migrations_paths.first
      end

      def migrations(paths)
        paths = Array(paths)

        files = Dir[*paths.map { |p| "#{p}/**/[0-9]*_*.rb" }]

        migrations = files.map do |file|
          version, name, scope = file.scan(/([0-9]+)_([_a-z0-9]*)\.?([_a-z0-9]*)?\.rb\z/).first

          raise IllegalMigrationNameError.new(file) unless version
          version = version.to_i
          name = name.camelize

          MigrationProxy.new(name, version, file, scope)
        end

        migrations.sort_by(&:version)
      end

      private

      def move(direction, migrations_paths, steps)
        migrator = self.new(direction, migrations(migrations_paths))
        start_index = migrator.migrations.index(migrator.current_migration)

        if start_index
          finish = migrator.migrations[start_index + steps]
          version = finish ? finish.version : 0
          send(direction, migrations_paths, version)
        end
      end
    end

    def initialize(direction, migrations, target_version = nil)
      raise StandardError.new("This database does not yet support migrations") unless Base.connection.supports_migrations?

      @direction         = direction
      @target_version    = target_version
      @migrated_versions = nil

      if Array(migrations).grep(String).empty?
        @migrations = migrations
      else
        ActiveSupport::Deprecation.warn "instantiate this class with a list of migrations"
        @migrations = self.class.migrations(migrations)
      end

      validate(@migrations)

      ActiveRecord::SchemaMigration.create_table
    end

    def current_version
      migrated.max || 0
    end

    def current_migration
      migrations.detect { |m| m.version == current_version }
    end
    alias :current :current_migration

    def run
      target = migrations.detect { |m| m.version == @target_version }
      raise UnknownMigrationVersionError.new(@target_version) if target.nil?
      unless (up? && migrated.include?(target.version.to_i)) || (down? && !migrated.include?(target.version.to_i))
        target.migrate(@direction)
        record_version_state_after_migrating(target.version)
      end
    end

    def migrate
      if !target && @target_version && @target_version > 0
        raise UnknownMigrationVersionError.new(@target_version)
      end

      running = runnable

      if block_given?
        message = "block argument to migrate is deprecated, please filter migrations before constructing the migrator"
        ActiveSupport::Deprecation.warn message
        running.select! { |m| yield m }
      end

      running.each do |migration|
        Base.logger.info "Migrating to #{migration.name} (#{migration.version})" if Base.logger

        begin
          ddl_transaction(migration) do
            migration.migrate(@direction)
            record_version_state_after_migrating(migration.version)
          end
        rescue => e
          canceled_msg = use_transaction?(migration) ? "this and " : ""
          raise StandardError, "An error has occurred, #{canceled_msg}all later migrations canceled:\n\n#{e}", e.backtrace
        end
      end
    end

    def runnable
      runnable = migrations[start..finish]
      if up?
        runnable.reject { |m| ran?(m) }
      else
        # skip the last migration if we're headed down, but not ALL the way down
        runnable.pop if target
        runnable.find_all { |m| ran?(m) }
      end
    end

    def migrations
      down? ? @migrations.reverse : @migrations.sort_by(&:version)
    end

    def pending_migrations
      already_migrated = migrated
      migrations.reject { |m| already_migrated.include?(m.version) }
    end

    def migrated
      @migrated_versions ||= Set.new(self.class.get_all_versions)
    end

    private
    def ran?(migration)
      migrated.include?(migration.version.to_i)
    end

    def target
      migrations.detect { |m| m.version == @target_version }
    end

    def finish
      migrations.index(target) || migrations.size - 1
    end

    def start
      up? ? 0 : (migrations.index(current) || 0)
    end

    def validate(migrations)
      name ,= migrations.group_by(&:name).find { |_,v| v.length > 1 }
      raise DuplicateMigrationNameError.new(name) if name

      version ,= migrations.group_by(&:version).find { |_,v| v.length > 1 }
      raise DuplicateMigrationVersionError.new(version) if version
    end

    def record_version_state_after_migrating(version)
      if down?
        migrated.delete(version)
        ActiveRecord::SchemaMigration.where(:version => version.to_s).delete_all
      else
        migrated << version
        ActiveRecord::SchemaMigration.create!(:version => version.to_s)
      end
    end

    def up?
      @direction == :up
    end

    def down?
      @direction == :down
    end

    # Wrap the migration in a transaction only if supported by the adapter.
    def ddl_transaction(migration)
      if use_transaction?(migration)
        Base.transaction { yield }
      else
        yield
      end
    end

    def use_transaction?(migration)
      !migration.disable_ddl_transaction && Base.connection.supports_ddl_transactions?
    end
  end
end