title: Order by relationship column
body: <p>I have the following query:</p>
  
  <pre><code>$items = UserItems::with('item')
          -&gt;where('user_id','=',$this-&gt;id)
          -&gt;where('quantity','&gt;',0)
          -&gt;get();
  </code></pre>
  
  <p>I need to order it by item.type so I tried:</p>
  
  <pre><code>$items = UserItems::with('item')
          -&gt;where('user_id','=',$this-&gt;id)
          -&gt;where('quantity','&gt;',0)
          -&gt;orderBy('item.type')
          -&gt;get();
  </code></pre>
  
  <p>but I get  <code>Unknown column 'item.type' in 'order clause'</code></p>
  
  <p>What I am missing?</p>