## application_helper.rb
module ApplicationHelper

  ACTIVE_MENU_CSS_CLASS = "class=\"active\""

  def css_class_from_action(controller, options={})
    if !options[:controller].blank? && !options[:action].blank?
      if controller.controller_name.downcase == options[:controller].downcase &&
        controller.action_name.downcase == options[:action].downcase
        ACTIVE_MENU_CSS_CLASS
      end
    elsif !options[:action].blank?
      if controller.action_name.downcase == options[:action].downcase
        ACTIVE_MENU_CSS_CLASS
      end
    elsif !options[:controller].blank?
      if controller.controller_name.downcase == options[:controller].downcase
        ACTIVE_MENU_CSS_CLASS
      end
    end

  end
end

## application_helper_spec.rb
require 'spec_helper'

describe ApplicationHelper do

  it "prints active CSS class in menu if controller matches" do
    controller_stub = mock("controller", :controller_name => "foo")
    css = helper.css_class_from_action(controller_stub, :controller => "foo")
    css.should == "class=\"active\""
  end

  it "prints active CSS class in menu if action matches" do
    controller_stub = mock("controller", :action_name => "bar")
    css = helper.css_class_from_action(controller_stub, :action => "bar")
    css.should == "class=\"active\""
  end

  it "prints active CSS class in menu if controller and action matches" do
    controller_stub = mock("controller", :controller_name => "foo", :action_name => "bar")
    css = helper.css_class_from_action(controller_stub, :controller => "foo", :action => "bar")
    css.should == "class=\"active\""
  end

  it "prints nothing if names don't match" do
    controller_stub = mock("controller", :controller_name => "foo", :action_name => "bar")
    css = helper.css_class_from_action(controller_stub, :controller => "bar", :action_name => "foo")
    css.should be_nil
  end

end

## example of usage
...

<li <%= css_class_from_action(controller, :action => "dashboard") %>><%= link_to "Dashboard", dashboard_path %></li>
<li <%= css_class_from_action(controller, :controller => "borrowings") %>><%= link_to "Borrowings", borrowings_path %></li>