title: Symfony and Doctrine - The identifier id is missing for a query
body: <p>I'm developping a website for a small business and I'm facing an unknown problem ...</p>
  
  <p>In the admin panel, I can manage my menus, contents etc... 
  For menus management, I succeeded</p>
  
  <pre><code>$form = $this-&gt;createFormBuilder($menu)
          -&gt;add('nom', 'text', array(
              "attr" =&gt; array(
                  'placeholder' =&gt; "Nom du menu",
                  'class'       =&gt; 'input-small'
              )
          ))
          -&gt;add('ordre', 'integer', array(
              "attr"  =&gt; array(
                  'placeholder'   =&gt; "Ordre",
                  "class"         =&gt; 'input-small'
              )
          ))
          -&gt;add('parent', 'choice', array(
              'placeholder'   =&gt; '-',
              'choices'   =&gt; $parents,
              'required'  =&gt; false
          ))
          -&gt;add('save', 'submit', array(
              'label'=&gt;'Ajouter',
              'attr'  =&gt; array(
                  'class' =&gt; 'btn'
              )
          ))
          -&gt;getForm();
  </code></pre>
  
  <p>I handle it correctly and it works. But now, I try it with another content to manage :</p>
  
  <pre><code>$blocInfoNew = new AccueilModif();
      $formNewInfo = $this-&gt;createFormBuilder($blocInfoNew)
          -&gt;add('titreBloc', 'text')
          -&gt;add('save', 'submit', array(
              "label"     =&gt; "Save"
          ))
          -&gt;getForm();
  [...]
  return $this-&gt;render("[...]:page_accueil.html.twig", array(
          'page'              =&gt; "Administration - Modifier la page d'accueil",
          [...]
          'formNewInfo'         =&gt; $formNewInfo-&gt;createView()
  ));
  </code></pre>
  
  <p>With a simple HTML form to test : </p>
  
  <pre><code>&lt;div class="modal hide fade" id="blocAddInfo"&gt;
      {{ form_start(formNewInfo) }}
          &lt;p class="f_legend"&gt;Ajouter une info&lt;/p&gt;
          {{ form_widget(formNewInfo.titreBloc) }}
          {{ form_widget(formNewInfo.save) }}
      {{ form_end(formNewInfo) }}
  &lt;/div&gt;
  </code></pre>
  
  <p>All works fine, until I send the form to the controller...</p>
  
  <blockquote>
    <p>The identifier id is missing for a query of [...]\AdminBundle\Entity\AccueilModif
    500 Internal Server Error - ORMException </p>
  </blockquote>
  
  <p>I searched all the internet without finding a solution >&lt; Can someone help me please ?</p>