观察者没有清除Rails 2.3.2中的缓存-请帮助

在我的Posts controller中,我有:

cache_sweeper Company::Caching::Sweepers::PostSweeper, :only => [:save_post]

我创建了以下模块:

module Company
  module Caching
    module Sweepers

      class PostSweeper < ActionController::Caching::Sweeper
        observe Post

        def after_save(post)
          Rails.cache.delete("post_" + post.permalink)
        end
      end

    end
  end
end