Trouble on caching an action when I rewrite a URL - ruby-on-rails

Question

I am using Ruby on Rails 3 and I have an issue on caching when I rewrite a URL in the model using the to_param method.
In my User model I have:
class User < ActiveRecord::Base
def to_param # Rewrites URL
"#{self.id}-#{self.name}-#{self.surname}"
end
...
end
In the User controller I have:
class UsersController < ApplicationController
caches_action :show
def show
...
end
end
In the Users sweeper I have:
class UsersSweeper < ActionController::Caching::Sweeper
observe User
def after_save(user)
clear_users_cache(user)
end
def after_destroy(user)
clear_users_cache(user)
end
def clear_users_cache(user)
expire_action :controller => :users, :action => :show, :id => user
end
end
Now, when I browse the user show page in the log file I get:
Write fragment views/<my_site_name>/users/2-Test_name-Test_surname (0.3ms)
When I expire the cache after a change the name or surname in the log file I get
Expire fragment views/<my_site_name>/users/2-New_test_name-New_test_surname (0.3ms)
So, since the data is changed, it doesn't expire the cache because Rails try to expire 2-New_test_name-New_test_surname and not 2-Test_name-Test_surname.
How can I "easly" handle the Rails caching behavior to make it to work?
P.S.: Of course if I don't use the to_param method, it works as well.
UPDATED
I can do something like this
caches_action :show, :cache_path => Proc.new { |c| 'users/' + c.params[:id].split('-').first }
but I don't think that is a good way to solve things...

Answer 1

Try using a custom path:
You can set modify the default action cache path by passing a :cache_path option. This will be passed directly to ActionCachePath.path_for. This is handy for actions with multiple possible routes that should be cached differently. If a block is given, it is called with the current controller instance.
caches_action :show, :cache_path => { :project => 1 }
Obviously customize to suit your needs. See the API for more info.