title: Embedded form entity is not persisted
body: <p>Symfony 2.6.9</p>
  
  <p>I have <code>UserProfile</code> entity with <code>avatar</code> property which has <code>oneToOne</code> uni-directional relation to <code>UserAvatar</code> entity and cascade persistence.</p>
  
  <pre><code>BlogBundle\Entity\UserProfile:
      type: entity
      table: user_profiles
      id:
          id:
              type: integer
              generator:
                  strategy: AUTO
      fields:
          #...
      oneToOne:
          #...
          avatar:
              targetEntity: UserAvatar
              cascade: ['persist', 'remove']
  </code></pre>
  
  <p>And I created <code>UserProfile</code> form type with embedded avatar form field:</p>
  
  <pre><code>    public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
              #...
              -&gt;add('avatar', new UserAvatarType, [
                  'required' =&gt; false,
              ])
              -&gt;add('submit', 'submit', [
                  'attr' =&gt; [
                      'class' =&gt; 'right'
                  ]
              ])
          ;
      }
  
      public function setDefaultOptions(OptionsResolverInterface $resolver)
      {
          $resolver-&gt;setDefaults(array(
              'data_class' =&gt; 'BlogBundle\Entity\UserProfile',
              'cascade_validation' =&gt; true,
          ));
      }
  
      public function getName()
      {
          return 'userProfile';
      }
  }
  </code></pre>
  
  <p>And <code>UserAvatarType</code>:</p>
  
  <pre><code>class UserAvatarType extends AbstractType
  {
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
              -&gt;add('file', 'file', [
                  'label' =&gt; false
              ])
          ;
      }
  
      public function setDefaultOptions(OptionsResolverInterface $resolver)
      {
          $resolver-&gt;setDefaults(array(
              'data_class' =&gt; 'BlogBundle\Entity\UserAvatar',
          ));
      }
  
      public function getName()
      {
          return 'userAvatar';
      }
  }
  </code></pre>
  
  <p><code>UserProfile</code> and <code>UserAvatar</code> entities already exist when I submit the form, they created automatically while user registration on persistence event. I just edit profile entity.</p>
  
  <p>When I submit form, all the fields inside <code>UserProfile</code> are persisted well, but not embedded <code>avatar</code> field. Method <code>setAvatar</code> inside <code>UserProfile</code> is not invoked. Validation on <code>UserAvatar</code> is passed well. No errors, <code>UserAvatar</code> entity just not persisted.</p>
  
  <p><a href="https://gist.github.com/iHedgehog/a92a64a196b66ce18b23" rel="nofollow">Gist.</a></p>
  
  <p>Can anybody help?</p>