assets_precompile_conditional.rb

namespace :deploy do
  namespace :assets do

    desc <<-DESC
      The task precompiles the Rails assets from the \
      Rails 3.1 asset pipeline *only* if any changes to \
      the assets have been made.
    DESC
    task :precompile, :roles => :web, :except => { :no_release => true } do
      from = source.next_revision(current_revision)
      if capture("cd #{latest_release} && #{source.local.log(from)} vendor/assets/ app/assets/ | 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