title: Using field in where
body: <p>I have a Model with two fields: <code>date1</code> and <code>date2</code>. I'd like to build a query which select data where <code>date2 + 10 &gt; date1</code>.  </p>
  
  <p><code>Model::where('date2', '&gt;', 'date1');
  </code></p>
  
  <p>works but not</p>
  
  <p><code>Model::where('date2', '&gt;', 'date1' + 10);
  </code></p>