title: Use a calculated column in a doctrine2 mapping definition
body: <p>Is it possible in doctrine2 to use a calculated column in the mapping definition? I have for example a table that has a column called <code>quantity</code> and a column <code>unit_price</code>. I want to add a calculated column <code>price</code> to the mapping which will be calculated by multiplying <code>quantity</code> and <code>unit_price</code>. I know that this will only be possible for SELECT statements.</p>
  
  <p>Could this be achieved using doctrine2?</p>