title: doctrine QueryBuilder arithmetic operation
body: <p>I am trying to do an arithmetic operation with doctrine Query Builder and assign the result to an alias.
  Following is an example from <a href="http://www.geeksengine.com/database/basic-select/arithmetic-operations.php" rel="nofollow">geeksengine</a> that I'd like to "convert" to doctrine QueryBuilder:
  </p>
  
  <pre><code>/*
  Query 1: This query calculates the total price before  discount and after discount for each order item.
  */
  SELECT order_id,
     product_id, 
     unit_price * quantity AS "regular_price", 
  FROM order_details;
  </code></pre>
  
  <p>This is what I started with:
  </p>
  
  <pre><code>$queryBuilder = $this-&gt;em-&gt;createQueryBuilder();
  $queryBuilder-&gt;select('o')
               -&gt;addSelect('o.unit_price * o.quantity AS regular_price')
               -&gt;from('Test\Model\Entity\OrderEntity', 'o');
  </code></pre>
  
  <p>But this throws an exception:</p>
  
  <pre><code>RuntimeException
  Not all identifier properties can be found in the ResultSetMapping: id
  </code></pre>
  
  <p>I guess, it's because <code>regular_price</code> is not part of the database - so it can't be mapped.</p>
  
  <p>I can get it done with pure DQL but for some reasons I have to stick with the QueryBuilder.</p>
  
  <p>Any ideas?</p>