model.rb

class Model
  
  def Model.new(*args, &block)
    allocate.instance_eval do
      @attributes = {}
      initialize(*args, &block)
      self
    end
  end

  attr(:attributes)

  def initialize(params = {})
    params.each do |key, val|
      @attributes[key.to_s] = val
    end

    @_in_method_missing = false
  end

  def method_missing(method, *args, &block)
    super if @_in_method_missing
    @_in_method_missing = true

    begin
      match, key, op = /^([^=?]+)([=?]?)$/.match(method.to_s).to_a

      case
        when op == '='
          val = args.shift
          attributes[key] = val

        when op == '?'
          attributes.has_key?(key)

        else
          if attributes.has_key?(key)
            attributes[key]
          else
            super
          end
      end
    ensure
      @_in_method_missing = false
    end
  end

  def inspect(*args, &block)
    "#{ type }(#{ attributes.inspect(*args, &block) })"
  end

  def type
    self.class.name
  end

  def serialize
    attributes.map{|k,v| [k,v].join('=')}.join(', ')
  end

end

class A < Model
end

class B < Model
end


a = A.new(:x => 0, :y => 1)
p a
p a.x
p a.y
a.x = 40
a.y = 2
p a.x
p a.y
p a.foobar?

puts

b = B.new(:x => 0, :y => 1)
p b
p b.x
p b.y


__END__

~> ruby model.rb

A({"x"=>0, "y"=>1})
0
1
40
2
false

B({"x"=>0, "y"=>1})
0
1