title: How to make a Symfony Callback validator Doctrine aware?
body: <p>As said in the title, I actually need to create a validation process with Symfony.
  I'm using YAML file, everything is okay.
  But in some cases, I need to check the database before saying that the data is validated.
  I was searching in the <a href="http://symfony.com/doc/current/reference/constraints/Callback.html" rel="nofollow">Callback</a> method, but it actually only allows me to basically check the values. I searched to make dependency injection, or even passing a defined service as a Callback, but it does not help too.</p>
  
  <p>So the question, in short is: is it possible to achieve it? In which way?</p>