Dynamic CSS with Rails

October 24th, 2008

I’ve sometimes wondered why CSS isn’t more dynamic.

Take, for example, the styles applied to the sub-menu on the services page of this site. To achieve the highlighting of the current service, I combine a Rails view and CSS as follows:

The Rails View

<div id="<%= @current_service.name %>">
  <div id="secondaryNav">
      <% @services.each do |service| %> 
        <li class="<%= service.name %>"><%= link_to service.heading, service_path(service) %></li>
      <% end %>


#agileweb #secondaryNav .agileweb a,
#java #secondaryNav .java a,
#coaching #secondaryNav .coaching a,
#mentoring #secondaryNav .mentoring a,
#overview #secondaryNav .overview a,
#advice #secondaryNav .advice a {
  color: #99CC00;
  text-decoration: underline;

Handling a new service

That does the trick nicely until I decide to add a new service. Wouldn’t it be nice to have that CSS regenerated dynamically whenever I create, update or delete a service?

Enter a new Ruby module:

# lib/dynamic_css.rb
module DynamicCss
  def generate_services_nav_links_css
    return if RAILS_ENV == "test"
    FileUtils.cd File.expand_path("public/stylesheets", RAILS_ROOT)
    File.open("servicesnav.css", "w") do |out|
      service_names = []
      services = Service.find :all
      services.each { |s| service_names << s.name }
      service_names.each_with_index do |name, i|
        out.print "##{name} #secondaryNav .#{name} a"
        if i + 1 < service_names.size
          out.puts ","
          out.puts " {"
      out.puts "  color: #99CC00;"
      out.puts "  text-decoration: underline;"
      out.puts "}"

Then a small adjustment to invoke the css regeneration via a filter in my admin services controller:

class Admin::ServicesController < AdminLayoutController
  include DynamicCss
  after_filter :generate_services_nav_links_css, :only => [:create, :update, :destroy]

# remainder of controller


Lastly, to ensure that the servicesnav.css file exists by the time one of the public services pages is requested:

# config/initializers/services_nav_css.rb

include DynamicCss


Admittedly this is a specific case, but this example shows that it is relatively straightforward to dynamically generate CSS within a Rails app if required.

Tags: css rails | Comments (3)