gistfile1.rb

class Factory
  attr_reader :username

  def initialize(args = {})
    @username = args.fetch :username
  end

  def self.register_class(classname)
    Factory.send :define_method, "#{classname.to_s.downcase}_by_name".to_sym do |args|
      classname.new({ :username => self.username}.merge args)
    end 
  end

  module Constructable
    def self.included(base)
      Factory::register_class(base)
    end
  end
end

module Base
  attr_accessor :username

  def initialize(args = {})
    self.username = args.fetch :username

    self.mandatory_attributes.each do |attribute_name, attribute_value|
      raise ArgumentError, "Mandatory attribute #{attribute_name} missing in initialization" if not args.has_key?(attribute_name)
      define_attr(attribute_name)
      self.send("#{attribute_name}=", args[attribute_name])
    end
  end

  def define_attr(sym, val = nil)
    singleton = class << self; self end

    singleton.send :define_method, "#{sym}=".to_sym do |val|
      self.instance_variable_set("@#{sym}", val)
    end

    singleton.send :define_method, sym.to_sym do
      self.instance_variable_get("@#{sym}")
    end
  end
end

class Host 
  include Base
  include Factory::Constructable

  def mandatory_attributes 
    [ :name, ]
  end
end

factory = Factory.new(:username => "lama")

host = factory.host_by_name(:name => "lama_by_name")
p host.name