title: Order two models by different attributes in one query
body: <pre><code>class Comment &lt; ActiveRecord::Base
    #  updated_at :datetime
    belongs_to :user
  end
  
  class Post &lt; ActiveRecord::Base
    #  last_edit_at :datetime
    belongs_to :user
  end
  </code></pre>
  
  <p>I want to query a certain user and display her comments and posts chronologically, based on her comments updated_at attribute and posts last_edit_at attribute respectively.</p>
  
  <p>I've tried with <a href="https://stackoverflow.com/a/6633436/3205492">an answer</a> from a similar question, but there the attributes are the same:</p>
  
  <pre><code>combined_sorted = (User.comments + User.likes).sort{|a,b| a.created_at &lt;=&gt; b.created_at }
  </code></pre>
  
  <p>How could I accomplish the above but with unique attributes?</p>