title: Symfony 2: Set field as read only after first save
body: <p>I have a Symfony 2 entity. When I create a new record, I must fill all the values using a form, but after saving it, one of the values, $amount shouldn't be updatable when I update the others members. </p>
  
  <p>How can I accomplish this? It's possible to mark a form member as a read-only, in runtime?</p>