title: Invalidate a specific model in the Rails cache
body: <p>I'm using Rails 3 with Memcached to cache some models. When the model changes, I want to invalidate the cache for that record. With view fragments, I just say <code>expire_fragment("blah")</code>. How do I do this with my models? I don't want to say <code>Rails.cache.clear</code> and lose the whole thing. I want something like <code>Rails.cache.invalidate("/users/5")</code>. How do I do that?</p>