base_controller.rb

require 'hyper_api'

class Api::BaseController < ApplicationController
  respond_to :json
  self.responder = Authentication::Responders::HyperApi
end

hyper_api.rb

module Authentication
  module Responders
    class HyperApi < ActionController::Responder

      # Default render for json
      #
      # Returns the rendered response.
      def to_json
        render :json => decorated(version).to_json
      end

      private

      # Figure out what version to use based on the Accept header in the format
      # of 'application/vnd.something.v1+json' which becomes 'v1'.
      # 
      # Returns the version of the request.
      def version
        match = @request.headers['Accept'].match(/\.(\w+)\+#{format}/)
        if match && version = match[1]
          version.capitalize
        else
          "v1"
        end
      end

      # Try and find a decorator class for the resource.
      #
      # version - The version of the decorater wanted.
      def decorator(version=nil)
        name = resource.class.name
        Object::const_get("#{name}#{version.to_s.capitalize}Decorator")
      rescue NameError
        nil
      end

      # Decorate the resource if possible.
      #
      # version - The version of the decorator.
      def decorated(version=nil)
        if decor = decorator(version)
          decor.decorate(resource)
        else
          resource
        end
      end

    end
  end
end

users_controller.rb

module Api
  class UsersController < BaseController
    def show
      user = User.find(params[:id])
      respond_with(user)
    end
  end
end