Volatile cache keys with Rails and Redis

Does Rails automatically set an EXPIRE attribute on keys when working
Redis, or will an old key sit there forever if it’s never manually
out? This is mostly a concern when using ActiveModel’s “cache_key”:

<%= cache @post do %>

The cache-key will be something like views/posts/744-20120613070958 .
the post is updated, and therefore gets a new cache-key, what happens to
the old one? Will it just sit there forever? Do I need to add an
expires_in option to every cache call, just to make sure it gets
up every once in a while?

<%= cache @post, expires_in: 1.month do %>


The idea is that your cache system should expire the oldest items in the
cache automatically when it hits its pre-configured size limit.