title: SonataAdminBundle $this->getSubject() is not working as expected
body: <p>Setup: symfony 2.0, SonataAdminBundle 2.0</p>
  
  <p>I have two entities. Company and Event. Company has many events.
  I also have an admin for Company which allows to edit company name, and associated events. </p>
  
  <p>In database I have 2 companies, and 3 events for each company.</p>
  
  <ol>
  <li>Company1
  <ul>
  <li>event1</li>
  <li>event2</li>
  <li>event3</li>
  </ul></li>
  <li>Company2
  <ul>
  <li>event4</li>
  <li>event5</li>
  <li>event6</li>
  </ul></li>
  </ol>
  
  <p><strong>CompanyAdmin</strong></p>
  
  <pre><code>protected function configureFormFields(FormMapper $formMapper)
  {
      $formMapper
          -&gt;add('name')
          -&gt;add('events', 'sonata_type_collection', array(), array(
              'edit'      =&gt; 'inline',
              'inline'    =&gt; 'table'
          ))
      ;
  
      $this-&gt;getFormFieldDescription('events')
          -&gt;setAssociationAdmin($this-&gt;getConfigurationPool()-&gt;getInstance('namespace.platform.admin.event_positions'));
  }
  </code></pre>
  
  <p><strong>EventPositionsAdmin</strong></p>
  
  <pre><code>protected function configureFormFields(FormMapper $formMapper)
  {
      $formMapper
          -&gt;add('name')
          -&gt;add('date')
      ;
  
      echo $this-&gt;getSubject().' ';
  }
  </code></pre>
  
  <p>The problem is <strong>echo $this->getSubject()</strong> for <strong>Company1</strong> prints <strong>event1 event1 event1</strong></p>
  
  <p>Expected result: <strong>event1 event2 event3</strong></p>