title: Sonata Admin - Order List by calculated value
body: <p>Is there a way to have a column in a Sonata Admin list that uses a calculated value as the sort by ??</p>
  
  <p>Something like this: ( this does NOT work )</p>
  
  <pre><code>    $mapper-&gt;add('stock', null, [
          'sortable' =&gt; '(stock.real - stock.inCustomerOrder)',
      ]);
  </code></pre>