title: Two form fields into one entity value
body: <p>How its possible to join two separated fields (must be separated) in one form (date and time for example) to one entity propery datetime for persisting after form post ?</p>
  
  <p>What is better way ? Data Transofmers ? Form events ? Form Model ? Manual setting all entity properties before persist ?</p>
  
  <p>Entity:</p>
  
  <pre><code>&lt;?php namespace Acme\DemoBundle\Entity;
  
  use Doctrine\ORM\Mapping as ORM;
  
  /**
   * @ORM\Entity
   * @ORM\Table(name="event")
   */
  class EventEntity
  {
  
      /**
       * @ORM\Column(name="id", type="integer", nullable=false)
       * @ORM\Id
       * @ORM\GeneratedValue(strategy="IDENTITY")
       */
      private $id;
  
      // ...
  
      /**
       * @ORM\Column(name="date_time", type="datetime", nullable=false)
       */
      protected $datetime;
  
      public function getId()
      {
          return $this-&gt;id;
      }
  
      // ...
  
      public function getDateTime()
      {
          return $this-&gt;datetime;
      }
  
      public function setDateTime(\DateTime $datetime)
      {
          $this-&gt;datetime = $datetime;
      }
  
  }
  </code></pre>
  
  <p>FormType:</p>
  
  <pre><code>&lt;?php namespace Acme\DemoBundle\Form\Type;
  
  use JMS\DiExtraBundle\Annotation as DI;
  use Symfony\Component\Form\AbstractType;
  use Symfony\Component\Form\FormBuilderInterface;
  use Symfony\Component\OptionsResolver\OptionsResolverInterface;
  
  class EventType extends AbstractType
  {
  
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
              // ...
              -&gt;add('date', 'date', [
                      'required' =&gt; true,
                      'widget'   =&gt; 'single_text',
                      'format'   =&gt; 'dd.MM.yyyy'
                  ]
              )
              -&gt;add('time', 'time', [
                      'required' =&gt; false,
                      'widget'   =&gt; 'single_text'
                  ]
              );
      }
  
      public function setDefaultOptions(OptionsResolverInterface $resolver)
      {
          $resolver-&gt;setDefaults(array(
              'data_class' =&gt; 'Acme\DemoBundle\Entity\EventEntity' //Acme\DemoBundle\Form\Model\EventModel ?
          ));
      }
  
      public function getName()
      {
          return 'event';
      }
  
  }
  </code></pre>