Best way to highlight current page in Rails 3? -- apply a css class to links conditionally

In app/helpers/application_helper.rb

def cp(path)
  "current" if current_page?(path)
end

In your views:

<%= link_to "All Posts", posts_path, class: cp(posts_path) %>

Basically write a simple wrapper around it. Additionally you could extend the method to allow additional classes to be applied by adding arguments. Keeps the views concise/dry. Or, without extending the method, you could just do simple String interpolation like so to add additional classes:

<%= link_to "All Posts", posts_path, class: "#{cp(posts_path)} additional_class" %>

In my case I have a lot of name spaced controllers, that is why I like to show if the current view also is in the Menu Path, I had use the solution of Michael van Rooijen and then I customize for my case.

Helper

def cp(path)
  "current" if request.url.include?(path)
end

View

<%= link_to "All Posts", posts_path, class: cp(posts_path) %>

Now if my menu bar is /users and my current page is /users/10/post also the link /users is set with "current" class

I branched off of Michael's answer and tweaked the helper:

def active_class?(*paths)
  active = false
  paths.each { |path| active ||= current_page?(path) }
  active ? 'active' : nil
end

Here's how you'd use it:

<%= link_to "Bookings", bookings_path, class: active_class?(bookings_path) %>

You can pass multiple paths to it in case you have a tab which could be rendered by multiple views:

<%= content_tag :li, class: active_class?(bookings_path, action: 'new') %>

And the great thing about this is if the conditions are false, it will insert nil. Why is this good? Well, if you provide class with nil it won't include the class attribute on the tag at all. Bonus!

In the interest of not having to repeat your self too much by having to check current_page inside the link_to method all the time, here's a custom helper that you can use (put this in app/views/helpers/application_helpers.rb

def link_to_active_class(name, active_class_names, options = {}, html_options = {}, &block)
  html_options[:class] = html_options[:class].to_s + active_class_names if current_page?(options.to_s)
  link_to name, options, html_options, &block
end

Example usage:

<div> <%= link_to_active_class('Dashboard', 'bright_blue', dashboard_path, class: 'link_decor') </div>

if you are on http://example.com/dashboard, then it should return:

<div> <a href='/dashboard' class='link_decor bright_blue'>Dashboard</a> </div>

Regards.