Help with caching (caches_page) in Rails 3

Rails
1

I code this on my controller:

caches_page :index
cache_sweeper :site_sweeper

https://github.com/leandromoreira/playground.activeadmin/blob/master/app/controllers/page_controller.rb

and this on my sweeper

class SiteSweeper < ActionController::Caching::Sweeper
observe Article, Category

def after_create(model)
  expire_cache_for(model)
end

def after_update(model)
  expire_cache_for(model)
end

def after_destroy(model)
  expire_cache_for(model)
end

private
def expire_cache_for(model)
  expire_page controller: 'page', action: 'index'
end

end

https://github.com/leandromoreira/playground.activeadmin/blob/master/app/sweepers/site_sweeper.rb

But I change my Article and the change do not expire the page, am I
doing something wrong?