title: Get last comment of all posts in one category
body: <p>Comment belongs to Post.<br>
  Post belongs to Category. </p>
  
  <p>How would I get a collection of every lastly updated comment for each post, all belonging to one single category?</p>
  
  <p>I've tried this but it just gives me one post:</p>
  
  <pre><code>category.posts.joins(:comments).order('updated_at DESC').first
  </code></pre>
  
  <p><strong>Update</strong></p>
  
  <p>What I want is to fetch <em>one</em> commment per post, the last updated comment for each post.</p>