How to cache an arbitrary object in Rails(time-based)?

I read the official guide. It says there are page cache, action cache and fragment cache, but they are not what I want.

I just like to cache an object, not the whole page or fragment of view, like this pseudocode:

def show  cache @ads, :expires_in => 1.hour do    @ads = Advertisement.all  doend

Is it possible? with memcache or redis?