assets.rake

require 'non_digest_assets'

namespace :assets do

  desc "Copy certain assets to a non-digest version"
  task :non_digested do
    NonDigestAssets.new.generate
  end

  # Run non_digested task after precompile
  Rake::Task['assets:precompile'].enhance do
    Rake::Task['assets:non_digested'].invoke
  end

end

non_digest_assets.rb

class NonDigestAssets
  include Rake::DSL

  FILES = ["external/app.js"]

  def generate
    latest_digested_files.map do |file|
      file = "public/assets/#{file}"
      non_digested = file.gsub /(\-[a-z0-9]{32})/, ""

      next unless FILES.any? {|s| non_digested =~ /#{s}/}
      copy(file, non_digested)
    end

  end

  def latest_digested_files
    manifest_path = Dir.glob(File.join(Rails.root, 'public/assets/manifest-*.json')).first
    manifest_data = JSON.load(File.new(manifest_path))

    manifest_data['assets'].map(&:second) # digested file names
  end

  def copy(from, to)
    sh "cp #{from} #{to}"
  end
end