Grape presenters?

gistfile1.rb

# lib/api/presenters/detailed_post.rb

module API
  module Presenters
    class DetailedPost
      include ActiveModel::Serializers::JSON
      include ActiveModel::Serializers::Xml
      
      attr_accessor :post
      delegate :attributes, :to => :post
      
      def initialize(post, *args)
        options = args.extract_options!
        
        @format = options.delete(:preprocess)
        @post   = post
      end
      
      def serializable_hash(options = nil)
        output = post.serializable_hash(options)
        output[:body]     = preprocess(output.delete(:body), @format)
        output[:comments] = post.comments.collect do |comment|
          preprocess(comment[:body], @format)
        end
        output
      end
      
      private
      
      def preprocess(content, format)
        case format
        when :markdown
          # transform markdown into html
        end
      end
    end
  end
end

# blog_api.rb

class BlogAPI < API::Grape
  presenter :detailed, :class => API::Presenters::DetailedPost
  
  get '/posts' do
    @posts = Post.where({:published => true})
    present @posts, :as => :detailed, :preprocess => :markdown
  end
end