gistfile1.rb

class Post
  class Mapper 
    include SomeMapper
    
    def self.find(id)
      post = super
      if post.published_at
        PublishedPost.new(post)
      else
        Draft.new(post)
      end
    end
    
    def self.save(derived_post)
      post = derived_post.__getobj__
      super(post)
    end
  end
end

class PublishedPost < SimpleDelegator
end

class Draft < SimpleDelegator
  def publish
    self.published_at = Time.now
    PublishedPost.new(__getobj__)
  end
end