title: Track field changes on Doctrine entity
body: <p>I want to track changes to a field of a Doctrine Entity. I use Symfony 2.5.0 and Doctrine 2.2.3.</p>
  
  <p>So far i have an <code>EventSubscriber</code> that subscribes to <code>preUpdate</code>. Here I want to create a new Entity which stores the new and old value and holds a reference to the Entity that is updated.</p>
  
  <p>The problem is, that I can't find a way to persist this new Entity. If I <code>persist()</code> in <code>preUpdate</code> and <code>flush()</code> in <code>postUpdate</code>, it works if I change only one Entity. If multiple Entities are changed, I get an error that the changeset is empty.</p>
  
  <p>I tried fiddling with different events with different results. Blank pages, tracking entites do not get persisted etc.</p>
  
  <p>I think that this should be a common use case - but I can't find examples.</p>