title: Deserializing doctrine references with JMSSerializer
body: <p>I'm not sure how to write the title properly since I'm not sure what I'm looking for, but basically what I'm wondering if there's a way to deserialize the following json:</p>
  
  <pre><code>{ "title": "hello world", "user": 1 }
  </code></pre>
  
  <p>But since <strong><em>user</em></strong> is a ORM mapping, attempt to find the entity from it's repository instead of trying to parse it directly.</p>
  
  <p>I can easily do this for a specific field by using a Handler for that case, pass along the EM and look the entity up, but I was hoping to come up with a more general solution where any relationship can be specified either by it's serialized fields (in order to create a new entity) or by it's id in order to lookup an existing one.</p>