title: How to call expire_fragment from Rails Observer/Model?
body: <p>I've pretty much tried everything, but it seems impossible to use 
  expire_fragment from models?  I know you're not supposed to and it's 
  non-MVC, but surely there much be some way to do it. </p>
  
  <p>I created a module in lib/cache_helper.rb with all my expire helpers, 
  within each are just a bunch of expire_fragment calls.  I have all my 
  cache sweepers setup under /app/sweepers and have an "include 
  CacheHelper" in my application controller so expiring cache within the 
  app when called via controllers works fine. </p>
  
  <p>Then things is I have some external daemons and especially some 
  recurring cron tasks which call a rake task that calls a certain 
  method.  This method does some processing and inputs entries into the 
  model, after which I need to expire cache.  </p>
  
  <p>What's the best way to do this as I can't specify cache sweeper within the model. 
  Straight up observers seem to be the best solution but then it 
  complains about expire_fragment being undefined etc etc, I've even 
  tried including the ActionController caching classes into the observer 
  but that didn't work.  I'd love some ideas of how to create a solution 
  for this.  Thanks. </p>