title: How to display Carrierwave cached thumbnail
body: <p>I use Carrierwave to allow users to attach images to their posts.  In the list of posts I display the thumbnails (in haml) as follows:</p>
  
  <pre><code>%td= image_tag post.image.url :thumb
  </code></pre>
  
  <p>If validation fails when creating a new post I display the cache like this:</p>
  
  <pre><code>= image_tag "/#{ImageUploader::cache_dir}/#{post.image_cache}"
  </code></pre>
  
  <p>What I can't figure out is how to display the cached thumbnail.  Checking the filesystem confirms that it lives in the same directory as the cached image.  I tried</p>
  
  <pre><code>= image_tag post.image_cache :thumb
  </code></pre>
  
  <p>but it errors with <code>wrong number of arguments (1 for 0)</code></p>