have `rake db:migrate` export the matching DDL to a file in db/migrate

config_initializers_export_migrations_as_ddl.rb

if $0.end_with?('/rake') && ARGV[0].start_with?('db:migrate') && Rails.env.development?

  #require 'active_record/migration'
  klass =
      case (adapter_class = ActiveRecord::Base.connection.class.to_s)
        when 'ActiveRecord::ConnectionAdapters::SQLite3Adapter'
          require 'active_record/connection_adapters/sqlite_adapter'
          ActiveRecord::ConnectionAdapters::SQLiteAdapter
        when 'ActiveRecord::ConnectionAdapters::Mysql2Adapter'
          require 'active_record/connection_adapters/mysql2_adapter'
          ActiveRecord::ConnectionAdapters::Mysql2Adapter
        when 'ActiveRecord::ConnectionAdapters::MysqlAdapter'
          require 'active_record/connection_adapters/mysql_adapter'
          ActiveRecord::ConnectionAdapters::MysqlAdapter
        else
          raise "Unsupported adapter : #{adapter_class}"
      end


  klass.class_eval('
    alias_method :original_execute, :execute

    def execute(sql, name = nil) #:nodoc:
      DDLMigrationsExporter.record_ddl(sql)
      original_execute(sql, name = nil)
    end
')

  class ActiveRecord::Migrator#:nodoc:
    class << self
      alias_method :original_migrate, :migrate

      def migrate(migrations_paths, target_version = nil, &block)
        migrations_summaries = original_migrate(migrations_paths, target_version, &block)
        DDLMigrationsExporter.export_ddl migrations_summaries
        migrations_summaries
      end
    end
  end

  class DDLMigrationsExporter
    class << self
      attr_accessor :migrations_as_ddl

      def record_ddl(sql)
        return if sql.include? '`schema_migrations`'
        return if sql.start_with?('SHOW')   \
               || sql.start_with?('SELECT') \
               || sql.start_with?('SET SQL_AUTO_IS_NULL=0')
        (self.migrations_as_ddl ||= []) << sql
      end

      def export_ddl(migrations_summaries)
        if migrations_summaries.empty?
          puts "\033[31mMIGRATIONS ARE UP-TO-DATE ->  NO DDL\033[0m"
          return
        end
        lines = []
        migrations_ids = migrations_summaries.collect(&:version)
        ddl_filename   = 'db/migrate/DDL_for_migrations_'   \
                       + migrations_ids.join('_')           \
                       + '.sql'

        lines << "-- DDL for the migrations :"
        lines += migrations_summaries.collect { |m|
          m.filename =~ /([^\/]+)$/
          "--              #{$1}"
        }
        lines << ''
        lines << "-- date : #{Time.now}"

        lines << ''
        lines << '-- DDL :'
        lines += migrations_as_ddl
        lines << '-- : LDD'

        puts "\033[32mExporting the DDL to : #{ddl_filename}\033[0m..."
        open(ddl_filename, 'w') do |f| lines.each do |ddl| f.puts ddl end end
        puts "\033[32mReading   the DDL file\033[0m :"
        File.open(ddl_filename).readlines.collect(&:chomp).each do |line|
          puts "\033[33m#{line}\033[0m".chomp
        end and puts
      end

    end
  end

end # if $0.end_with?('/rake') && ARGV[0].start_with?('db:migrate') && Rails.env.development?