[Rails] Volatile cache keys with Rails and Redis

Does Rails automatically set an EXPIRE attribute on keys when working with Redis, or will an old key sit there forever if it's never manually cleared 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 . When 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 cleaned up every once in a while?

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

Thanks,

Bryan

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