better_precompile.rb

# Sprockets no longer creates undigested assets, so we have to emulate the old behavior to keep vendored JS libraries happy.
# This automatically uses the newest asset in cases where the same file exists multiple times (like with Capistrano).
#
# https://github.com/rails/sprockets-rails/issues/49
#
# digested: 4-star-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.jpg
# undigested: 4-star.jpg
assets_without_digest = -> do
  digest = /(-{1}[a-z0-9]{64}*\.{1}){1}/
  assets = Dir.glob File.join Rails.root, 'public/assets/**/*'

  assets.reject! do |file|
    File.directory?(file) || file !~ digest || ['.json', '.map'].include?(File.extname(file))
  end

  assets.group_by{ |file| file.gsub digest, '.' }.each do |undigested_file, files|
    newest_file = files.max_by{ |file| File.ctime file }

    FileUtils.cp newest_file, undigested_file
  end
end

# Sprockets generates source maps, but it only tells browsers about their existence in local dev.
# This patch makes it so that statically compiled assets link their the source map.
#
# https://github.com/rails/sprockets/issues/502
#
# Given an application.js.coffee source file, Sprockets generates these files:
#
# minified source:  application-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.js
# source map:       application.js-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb.map
# unminifed source: application.source-cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc.coffee
add_source_map_url = -> do
  source_maps = Dir.glob File.join Rails.root, 'public/assets/*.map'
  source_maps.each do |source_map|
    /(?<name>\w+)\.(?<ext>\w+)-/ =~ source_map

    Dir.glob(File.join(Rails.root, "public/assets/#{name}-*.#{ext}")).each do |served_file|
      next if `tail -n 1 #{served_file}`.include? 'sourceMappingURL'

      comment = case ext
      when 'js'
        "\n//# sourceMappingURL=#{File.basename(source_map)}"
      when 'css'
        "\n/*# sourceMappingURL=#{File.basename(source_map)} */"
      end

      file = File.open served_file, 'a'
      file.write comment
      file.close
    end
  end
end

task 'assets:precompile' => 'environment' do
  assets_without_digest.call
  add_source_map_url.call
end