application_controller.rb

# Hooks into ApplicationController's write_fragment method.
# write_fragment is used for action and fragment caching.
# Create an alias method chain to call our customer method
# which stores the associated key with the tag in a 
# Redis Set. Then we can expire all those keys from anywhere
# in the code using Rails.cache.delete
#
# I use alias_method_chain instead of calling 'super'
# because there is a very rare case where someone 
# may have redfined 'write_fragment' in their own 
# controllers. Using an alias method chain
# keeps those methods intact.

class ApplicationController < ActionController::Base
  def write_fragment_with_tagged_key(key, content, options = nil)
    if options && options[:tag] && Cashier.perform_caching? 
      tags = case options[:tag].class.to_s
             when 'Proc', 'Lambda'
               options[:tag].call(self)
             else 
               options[:tag]
             end
      Cashier.store_fragment fragment_cache_key(key), *tags
    end
    write_fragment_without_tagged_key(key, content, options)
  end
  alias_method_chain :write_fragment, :tagged_key
end