perform precompile only when any of the asset files has changed since the last deploy

deploy.rb

# do not shallow_clone from Git repo

namespace :deploy do
  namespace :assets do
    # perform precompile only when any of the asset files has changed since the last deploy
    task :precompile, :roles => :web, :except => {:no_release => true} do
      from = source.next_revision(current_revision)
      asset_changing_files = ['vendor/assets/', 'app/assets/', 'lib/assets', 'Gemfile', 'Gemfile.lock'].select {|f| File.exists? f}

      if capture("cd #{latest_release} && #{source.local.log(from)} #{asset_changing_files.join(' ')} | wc -l").to_i > 0
        run %Q{cd #{latest_release} && #{rake} RAILS_ENV=#{rails_env} #{asset_env} assets:precompile}
      else
        logger.info 'Skipping asset pre-compilation because there were no asset changes'
      end
    end
  end
end