snippet.rb

module AttributeBuilder
  def initialize(attrs = {})
    attrs ||= {}
    attrs.each do |key, value|
      self.send("#{key}=", value)
    end
  end
end

module MemberBuilder
  def self.included(base)
    base.extend ClassMethods
  end
  
  module ClassMethods
    def buildable_member(member, clazz)
      define_method "#{member}=" do |attr|
        case attr
          when clazz then instance_variable_set("@#{member}", attr)
          else instance_variable_set("@#{member}", clazz.new(attr))
        end
      end
    end
  end
end

class Address
  include AttributeBuilder
  
  attr_accessor :street, :city
end

class Person
  include AttributeBuilder
  include MemberBuilder
  
  attr_accessor :name, :gender, :address
  buildable_member :address, Address
end

a = { :name => "Pencho", :gender => "M", :address =>  { :street => "Elm", :city => "Anaheim" }}
p = Person.new(a)
puts p.address.city