title: Symfony 2.1 + @MongoDBUnique(fields="email") not unique
body: <p>I'm trying to use <strong>MongoDB</strong> with <strong>Symfony2.1</strong> but I'm facing a problem.
  I'm following the <a href="http://symfony.com/doc/current/bundles/DoctrineMongoDBBundle/form.html" rel="nofollow">tutorial</a> from Symfony website called "<em>How to implement a simple Registration Form with MongoDB</em>".</p>
  
  <p>In this tutorial, it is said that we can add a constraint on email field as a unique field.
  After some tests, it doesn't work. I can add more than once the same email for my User entity.</p>
  
  <p>The annotation <em>@MongoDBUnique(fields="email")</em> seems to fail.</p>
  
  <p>I found someone which has the same <a href="https://groups.google.com/forum/?fromgroups#!topic/doctrine-dev/yxPtk-gRghw" rel="nofollow">problem</a> in a Google Group. At this time (posted on january, 6th), no one has answered or no one seems to have any answer unfortunelately.</p>
  
  <p>Thanks for any clue !</p>