gistfile1.rb

ActiveRecord::ConnectionAdapters::PostgreSQLColumn.class_eval do
  prepend Module.new {
    def mpq?
      type == :mpq
    end
  
    def simplified_type(field_type)
      field_type == 'mpq' ? :mpq : super
    end

    def klass
      mpq? ? Rational : super
    end

    def type_cast(string)
      if mpq?
        string.nil? ? nil : string.to_r
      else
        super
      end
    end

    def type_cast_code(var_name)
      mpq? ? "#{var_name}.to_r" : super
    end
  }

  class << self
    prepend Module.new {
      def extract_value_from_default(default)
        default =~ /\A'(.*)'::mpq\z/ ? $1 : super
      end
    }
  end
end