title: two fields in select field form Symfony2
body: <p>To create one appointment I need to choice one patient.
  In select list I need to show the name and age of patients.</p>
  
  <p>The code:</p>
  
  <pre><code>class AppointmentType extends AbstractType{
  
  public function buildForm(FormBuilder $builder, array $options)
  {
      $builder-&gt;add('id', 'entity', array('class'=&gt;'ProjectAppointmentBundle:Patient',
              'label'=&gt;'Patient '
          ));
  
  }
  
  public function getName()
  {
      return 'patient';
  }
  </code></pre>
  
  <p>}</p>
  
  <p>and the action:</p>
  
  <pre><code>public function newAction()
  {
      ...
  
      $patient = new Patient();
  
      $entity = new Appointment($patient);
      $form   = $this-&gt;createForm(new AppointmentType(), $entity);
  
      return array(
              'entity' =&gt; $entity,
              'form'   =&gt; $form-&gt;createView()
      );
  }
  </code></pre>
  
  <p>That is working but show only the name of patient (_toString() method return the name of patient). 
  I need show the name and age of patient in select field.</p>
  
  <p>Entity Patient have id,name,age,adress,etc...</p>
  
  <p>Someone can help me?</p>