retrieve_cache_key

# File activesupport/lib/active_support/cache.rb, line 92
        def retrieve_cache_key(key)
          case
          when key.respond_to?(:cache_key) then key.cache_key
          when key.is_a?(Array)            then key.map { |element| retrieve_cache_key(element) }.to_param
          when key.respond_to?(:to_a)      then retrieve_cache_key(key.to_a)
          else                                  key.to_param
          end.to_s
        end