title: Sonata Admin Bundle One-to-Many relationship not saving foreign ID
body: <p>I have a problem with the SonataAdminBunle in combination with symfony 2.2.
  I have a Project entity and a ProjectImage entity and specified a One-to-Many relationship between these two like so:</p>
  
  <pre><code>class Project
  {
      /**
       * @ORM\OneToMany(targetEntity="ProjectImage", mappedBy="project", cascade={"all"}, orphanRemoval=true)
       */
      private $images;
  }
  
  class ProjectImage
  {
  
      /**
       * @ORM\ManyToOne(targetEntity="Project", inversedBy="images")
       * @ORM\JoinColumn(name="project_id", referencedColumnName="id")
       */
      private $project;
  }
  </code></pre>
  
  <p>I've configured the ProjectAdmin and ProjectImageAdmin:</p>
  
  <pre><code>class ProjectAdmin extends Admin
  {
      protected function configureFormFields(FormMapper $formMapper)
      {
          $formMapper
              -&gt;add('title')
              -&gt;add('website')
              -&gt;add('description', 'textarea')
              -&gt;add('year')
              -&gt;add('tags')
              -&gt;add('images', 'sonata_type_collection', array(
                              'by_reference' =&gt; false
              ), array(
                              'edit' =&gt; 'inline',
                              'inline' =&gt; 'table',
                              'sortable' =&gt; 'id',
              ))
              ;
      }
  }
  
  class ProjectImageAdmin extends Admin
  {
      protected function configureFormFields(FormMapper $formMapper)
      {
          $formMapper
              -&gt;add('file', 'file', array(
                            'required' =&gt; false
              ))
              ;
      }
  }
  </code></pre>
  
  <p>The problem is that in the project_image table in the database the project_id is not saved, while all other data is and also the image is saved. Couldn't find a working answer anywhere else.</p>