carrierwave.rb

# config/initializers/carrierwave.rb
CarrierWave.configure do |config|
  config.grid_fs_database = "database_name"
  config.grid_fs_host = 'localhost'
  config.grid_fs_access_url = "/uploads"
  config.storage = :grid_fs
end

gridfs_serve_image.rb

# app/metal/gridfs_serve_image.rb
require 'mongo'

require(File.dirname(__FILE__) + "/../../config/environment") unless defined?(Rails)

class GridfsServeImage
  def self.call(env)
    if env["PATH_INFO"] =~ /^\/uploads\/(.+)$/
      begin
        Mongo::GridFileSystem.new(Mongoid.database).open($1, 'r') do |file|
          [200, { 'Content-Type' => file.content_type }, [file.read]]
        end
      rescue
        [404, { 'Content-Type' => 'text/plain' }, ['File not found.']]
      end
    else
      [404, { "Content-Type" => "text/html", "X-Cascade" => "pass" }, ["Not Found"]]
    end
  end
end

show.html.haml

-# app/views/something/show.html.haml
= image_tag @something.photo.url