common.rb

module DmSkinnySpec::Validations

  module Common

    class ExpectationArgs
      
      attr_reader :attribute, :context

      def initialize( attribute, options )
        @attribute = attribute
        @context   = options.delete(:when) || :default
        @message   = options.delete(:message)
      end
      
      protected
      
        def default_message( type, *values )
          DataMapper::Validate::ValidationErrors::default_error_message( type, attribute, *values )
        end
      
        def suggested_message
          @message
        end 
          
    end

end

it_should_validate_present.rb

 module DmSkinnySpec::Validations::ItShouldValidatePresent

  class ExpectationArgs < DmSkinnySpec::Validations::Common::ExpectationArgs
          
    def initialize( attribute, options )
      @type = options.delete(:type) || :String
      super 
    end
      
    def message
      suggested_message ||
        case @type
          when :Boolean ; default_message(:nil)
          else          ; default_message(:blank)
        end
    end

    def bad_values
      @values ||=
        case @type
          when :String ; [ nil, '' ]
          else         ; [ nil ]
        end
    end

  end

end