title: Symfony 2 Entity field type with select and/or add new
body: <h2>Context:</h2>
  
  <p>Let there be two entities (correctly mapped for Doctrine).</p>
  
  <ol>
  <li><code>Post</code> with properties {<code>$id</code> (integer, autoinc), <code>$name</code> (string), <code>$tags</code> (collection of <code>Tag</code>)}</li>
  <li><code>Tag</code>  with properties {<code>$id</code> (integer, autoinc), <code>$name</code> (string), <code>$posts</code> (collection of <code>Post</code>)}</li>
  </ol>
  
  <p>Relationship between these two is <code>Many-To-Many</code>.</p>
  
  <h2>Problem:</h2>
  
  <p>When creating a new <code>Post</code>, I want to immediately add tags to it. </p>
  
  <p>If I wanted to add <code>Tags</code> that already are peristed, I would create <a href="http://symfony.com/doc/current/reference/forms/types/entity.html">entity field type</a>, no problem with that.</p>
  
  <p><strong>But</strong> what would I do, if I wanted to add completely new <code>Tags</code> too? (Check some of already existing tags, fill name for new tag, maybe add some another new tag, then after submit assign everyting properly to <code>Post</code> entity)</p>
  
  <pre>
      Create new Post:
       Name: [__________]
  
      Add tags
      |
      |[x] alpha
      |[ ] beta
      |[x] gamma
      |
      |My tag doesnt exist, create new:
      |
      |Name: [__________]
      |
      |+Add another new tag
  </pre> 
  
  <p>Is there any way to do this? I know the basics of Symfony 2, but have no idea how to deal with this. Also surprised me I havent found my answer anywhere, seems like a common problem to me. What am I missing?</p>