migration.rb

module ActiveRecord
  class IrreversibleMigration < ActiveRecordError#:nodoc:
  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

  # 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
  #
  # * <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>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_name)</tt>: Removes the column named +column_name+ 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> and <tt>:unique</tt> (e.g. <tt>{ :name => "users_name_index", :unique => true }</tt>).
  # * <tt>remove_index(table_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 
  #   script/generate migration MyNewMigration
  #
  # where MyNewMigration is the name of your migration. The generator will
  # create an empty migration file <tt>nnn_my_new_migration.rb</tt> in the <tt>db/migrate/</tt>
  # directory where <tt>nnn</tt> is the next largest migration number.
  #
  # You may then edit the <tt>self.up</tt> and <tt>self.down</tt> 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 <tt>nnn_add_fieldname_to_tablename</tt>, 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
  # <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 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
  # <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 environment.rb.
  #
  class Migration
    @@verbose = true
    cattr_accessor :verbose

    class << self
      def up_with_benchmarks #:nodoc:
        migrate(:up)
      end

      def down_with_benchmarks #:nodoc:
        migrate(: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

        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

      # Because the method added may do an alias_method, it can be invoked
      # recursively. We use @ignore_new_methods as a guard to indicate whether
      # it is safe for the call to proceed.
      def singleton_method_added(sym) #:nodoc:
        return if defined?(@ignore_new_methods) && @ignore_new_methods

        begin
          @ignore_new_methods = true

          case sym
            when :up, :down
              klass = (class << self; self; end)
              klass.send(:alias_method_chain, sym, "benchmarks")
          end
        ensure
          @ignore_new_methods = false
        end
      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
        ActiveRecord::Base.connection
      end

      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
    end
  end

  # MigrationProxy is used to defer loading of the actual migration classes
  # until they are needed
  class MigrationProxy

    attr_accessor :name, :version, :filename

    delegate :migrate, :announce, :write, :to=>:migration

    private

      def migration
        @migration ||= load_migration
      end

      def load_migration
        load(filename)
        name.constantize
      end

  end

  class Migrator#:nodoc:
    class << self
      def migrate(migrations_path, target_version = nil)
        case
          when target_version.nil?              then up(migrations_path, target_version)
          when current_version == 0 && target_version == 0 then # noop
          when current_version > target_version then down(migrations_path, target_version)
          else                                       up(migrations_path, target_version)
        end
      end

      def rollback(migrations_path, steps=1)
        migrator = self.new(:down, migrations_path)
        start_index = migrator.migrations.index(migrator.current_migration)
        
        return unless start_index
        
        finish = migrator.migrations[start_index + steps]
        down(migrations_path, finish ? finish.version : 0)
      end

      def up(migrations_path, target_version = nil)
        self.new(:up, migrations_path, target_version).migrate
      end

      def down(migrations_path, target_version = nil)
        self.new(:down, migrations_path, target_version).migrate
      end
      
      def run(direction, migrations_path, target_version)
        self.new(direction, migrations_path, target_version).run
      end

      def migrations_path
        'db/migrate'
      end

      def schema_migrations_table_name
        Base.table_name_prefix + 'schema_migrations' + Base.table_name_suffix
      end

      def get_all_versions
        Base.connection.select_values("SELECT version FROM #{schema_migrations_table_name}").map(&: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 proper_table_name(name)
        # Use the Active Record objects own table_name, or pre/suffix from ActiveRecord::Base if name is a symbol/string
        name.table_name rescue "#{ActiveRecord::Base.table_name_prefix}#{name}#{ActiveRecord::Base.table_name_suffix}"
      end
    end

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

    def current_version
      migrated.last || 0
    end
    
    def current_migration
      migrations.detect { |m| m.version == current_version }
    end
    
    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
      current = migrations.detect { |m| m.version == current_version }
      target = migrations.detect { |m| m.version == @target_version }

      if target.nil? && !@target_version.nil? && @target_version > 0
        raise UnknownMigrationVersionError.new(@target_version)
      end
      
      start = up? ? 0 : (migrations.index(current) || 0)
      finish = migrations.index(target) || migrations.size - 1
      runnable = migrations[start..finish]
      
      # skip the last migration if we're headed down, but not ALL the way down
      runnable.pop if down? && !target.nil?
      
      runnable.each do |migration|
        Base.logger.info "Migrating to #{migration.name} (#{migration.version})"

        # On our way up, we skip migrating the ones we've already migrated
        next if up? && migrated.include?(migration.version.to_i)

        # On our way down, we skip reverting the ones we've never migrated
        if down? && !migrated.include?(migration.version.to_i)
          migration.announce 'never migrated, skipping'; migration.write
          next
        end

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

    def migrations
      @migrations ||= begin
        files = Dir["#{@migrations_path}/[0-9]*_*.rb"]
        
        migrations = files.inject([]) do |klasses, file|
          version, name = file.scan(/([0-9]+)_([_a-z0-9]*).rb/).first
          
          raise IllegalMigrationNameError.new(file) unless version
          version = version.to_i
          
          if klasses.detect { |m| m.version == version }
            raise DuplicateMigrationVersionError.new(version) 
          end

          if klasses.detect { |m| m.name == name.camelize }
            raise DuplicateMigrationNameError.new(name.camelize) 
          end
          
          klasses << (MigrationProxy.new).tap do |migration|
            migration.name     = name.camelize
            migration.version  = version
            migration.filename = file
          end
        end
        
        migrations = migrations.sort_by(&:version)
        down? ? migrations.reverse : migrations
      end
    end

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

    def migrated
      @migrated_versions ||= self.class.get_all_versions
    end

    private
      def record_version_state_after_migrating(version)
        sm_table = self.class.schema_migrations_table_name

        @migrated_versions ||= []
        if down?
          @migrated_versions.delete(version.to_i)
          Base.connection.update("DELETE FROM #{sm_table} WHERE version = '#{version}'")
        else
          @migrated_versions.push(version.to_i).sort!
          Base.connection.insert("INSERT INTO #{sm_table} (version) VALUES ('#{version}')")
        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(&block)
        if Base.connection.supports_ddl_transactions?
          Base.transaction { block.call }
        else
          block.call
        end
      end
  end
end