application.rb

require 'sinatra/base'
require 'sinatra/flash'
require 'rack/csrf'
require 'haml'
require 'sass'
require 'compass'

class Application < Sinatra::Application
  register Sinatra::Flash

  set :root, File.dirname(__FILE__)
  set :sprockets, (Sprockets::Environment.new(root) { |env| env.logger = Logger.new(STDOUT) })
  set :assets_prefix, 'compiled'
  set :assets_path, File.join(root, 'public', assets_prefix)
  set :compass_gem_root, Gem.loaded_specs['compass'].full_gem_path

  configure do
    sprockets.append_path File.join(root, 'assets', 'stylesheets')
    sprockets.append_path File.join(compass_gem_root, 'frameworks', 'compass', 'stylesheets')
    sprockets.append_path File.join(compass_gem_root, 'frameworks', 'blueprint', 'stylesheets')
    sprockets.append_path File.join('assets', 'javascripts')
   
    # ... more config
  end

  # we are deploying to heroku, which does not have a JVM, which YUI needs, so let's
  # only require and config the compressors / minifiers for dev env
  configure :development do
    require 'yui/compressor'
    require 'uglifier'
    sprockets.css_compressor = YUI::CssCompressor.new
    sprockets.js_compressor  = Uglifier.new(mangle: true)
  end

  # ... more setup
end

asset_helpers.rb

require 'sinatra/base'

module Sinatra
  module AssetHelpers
    # +options+ can pass "force_production" to trump env setting
    # if digest is used in the future, will need to incorporate this into these two helpers
    def stylesheets_tag(options = {})
      if production? || options[:force_production]
        '<link rel="stylesheet" href="/compiled/css/application.min.css" type="text/css" media="all" />'
      else
        '<link rel="stylesheet" href="/assets/application.css" type="text/css" media="all" />'
      end
    end

    def javascripts_tag(options = {})
      if production? || options[:force_production]
        '<script src="/compiled/js/application.min.js" type="text/javascript"></script>'
      else
        '<script src="/assets/application.js" type="text/javascript"></script>'
      end
    end

    helpers AssetHelpers
  end
end

Rakefile

require 'rubygems'
require 'bundler'

Bundler.require
require './application'

namespace :assets do
  desc 'compile assets'
  task :compile => [:compile_js, :compile_css] do
  end

  desc 'compile javascript assets'
  task :compile_js do
    sprockets = Application.settings.sprockets
    asset     = sprockets['application.js']
    outpath   = File.join(Application.settings.assets_path, 'js')
    outfile   = Pathname.new(outpath).join('application.min.js') # may want to use the digest in the future?

    FileUtils.mkdir_p outfile.dirname

    asset.write_to(outfile)
    asset.write_to("#{outfile}.gz")
    puts "successfully compiled js assets"
  end

  desc 'compile css assets'
  task :compile_css do
    sprockets = Application.settings.sprockets
    asset     = sprockets['application.css']
    outpath   = File.join(Application.settings.assets_path, 'css')
    outfile   = Pathname.new(outpath).join('application.min.css') # may want to use the digest in the future?

    FileUtils.mkdir_p outfile.dirname

    asset.write_to(outfile)
    asset.write_to("#{outfile}.gz")
    puts "successfully compiled css assets"
  end
  # todo: add :clean_all, :clean_css, :clean_js tasks, invoke before writing new file(s)
end