title: Rails map unique values and order in same statement
body: <p>The following statement identifies all the unique items</p>
  
  <pre><code>@products = @volumes.map(&amp;:product).uniq
  </code></pre>
  
  <p>However, in view of further processing, the goal is to order the items as well.  The following type of statement will not process because <code>undefined method 'order' for #&lt;Array:</code></p>
  
  <pre><code>@products = @volumes.map(&amp;:product).order("max_units DESC").uniq
  </code></pre>
  
  <p>How can this be accomplished succinctly?</p>