engine.rb

module SpliceReports
  class Engine < Rails::Engine

    initializer :register_assets do |engine|

      if Rails.env.production?
        assets = YAML.load_file("#{SpliceReports::Engine.root}/public/assets/manifest.yml")

        assets.each_pair do |file, digest|
          engine.config.assets.digests[file] = digest
        end
      end

      engine.middleware.use ::ActionDispatch::Static, "#{SpliceReports::Engine.root}/public"
    end

  end
end

Rakefile

desc 'Compile stand alone engine assets'
task 'assets:precompile:engine' do
  require 'sprockets'
  require 'sprockets/railtie'
  require 'uglifier'
  require 'sass/rails/compressor'
  require File.expand_path('../lib/splice_reports', __FILE__)

  precompile = [
    'splice_reports/splice.css',
    'splice_reports/splice_reports.js'
  ]

  env = Sprockets::Environment.new(SpliceReports::Engine.root)
  env.js_compressor = Uglifier.new
  env.css_compressor = Sass::Rails::CssCompressor.new

  paths = [
    'app/assets/stylesheets',
    'app/assets/javascripts'
  ]

  paths.each do |path|
    env.append_path(path)
  end

  target   = File.join(SpliceReports::Engine.root, 'public', 'assets')
  compiler = Sprockets::StaticCompiler.new(env,
                                           target,
                                           precompile,
                                           :manifest_path => File.join(target),
                                           :digest => true,
                                           :manifest => true)
  compiler.compile
end