When creating a new Rails 3.0 project and including firewatir and commonwatir in the list of gems an error is thrown when accessing a controller. Here are the repro steps:

gem install rails
gem install firewatir
gem install commonwatir

> rails new testrails
> cd testrails
> echo 'gem "commonwatir"' >> Gemfile
> echo 'gem "firewatir"' >> Gemfile

> bundle install --deployment

> rails generate controller Pages home

> rails server

From a web browser request the page http://localhost.local:3000/pages/home

The following error is thrown: NoMethodError: undefined method `cache' for :active_support:Symbol

I tracked this down to this file:

commonwatir-1.8.0/lib/watir/core_ext.rb

def underscore
gsub(/\B[A-Z][^A-Z]/, '\&').downcase.gsub(' ', '')
end

The underscore method is added/redefined in the core String class, which Rails also redefines using ActiveSupport::Inflector, but the underscore methods are incompatible. If that method is commented out, the above error is no longer thrown. I'm not sure of a good solution to this. Either rename the method and refactor all the watir code or depend ActiveSupport.