y_n_boolean.rb

module DataMapper
  class Property
    class YNBoolean < Boolean
      primitive ::String # otherwise adapters won't be able to correctly store the value

      def length
        @length ||= 1
      end

      def valid?(value, negated = false)
        dumped_value = dump(value)

        ret = (dumped_value == 'Y' || dumped_value == 'N') || (dumped_value.nil? && !required?)

        negated ? !ret : ret
      end

      def dump(value)
        value ? 'Y' : 'N'
      end

      def load(value)
        typecast(value)
      end

      def typecast(value)
        if value == 'Y'
          true
        elsif value == 'N'
          false
        else
          super
        end
      end
    end
  end
end

y_n_boolean_spec.rb

describe DataMapper::Property::YNBoolean do
  supported_by :all do
    before :all do
      class ::Post
        include DataMapper::Resource

        property :id,     Serial
        property :published, YNBoolean

        auto_migrate!
      end
    end

    it 'should work with Boolean' do
      post = ::Post.new :published => true

      post.save.should be(true)
      post.reload.published.should be(true)
    end

    it 'should work with Y' do
      post = ::Post.new :published => 'Y'

      post.save.should be(true)
      post.reload.published.should be(true)
    end

    it 'should work with N' do
      post = ::Post.new :published => 'N'

      post.save.should be(true)
      post.reload.published.should be(false)
    end
  end
end