Should a method ending in ? (question mark) return only a boolean?

A method ending with ? should return a value which can be meaningfully evaluated to true or false. If you want to ensure a boolean return, you can do so by adding a double bang to the finder.

def is_subscribed?(feed_url)
  !!Subscription.find_by_user_id_and_feed_id(self[ :id ], Feed.find_by_feed_url(feed_url))
end