title: Symfony2 Expected argument of type "object or array", "string" given
body: <p>I'm trying desperately to save the form with entity City OneToMany Anagrafic.
  I entered the "property_path" CityType the form and I returned error,</p>
  
  <pre><code>Expected argument of type “object or array”, “string” given
  </code></pre>
  
  <p>I do not understand what I'm doing wrong!</p>
  
  <pre><code>class Anagrafic
  {
  /**
   * @ORM\ManyToOne(targetEntity="City", inversedBy="anagrafics", cascade={"persist"})
   * @ORM\JoinColumn(name="city_id", referencedColumnName="id")
  */
  private $city;
  //..
  //..
  class City
  {
  /**
   * @ORM\OneToMany(targetEntity="Anagrafic", mappedBy="city", cascade={"persist"})
   */
  private $anagrafics;
  //...
  //...
  class CityType extends AbstractType
  {
  public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder-&gt;add('comune', 'hidden', array('property_path' =&gt; 'city.id'))
  //..
  //..
  class AnagraficType extends AbstractType
  {
  public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder-&gt;add('city', new CityType())
  </code></pre>
  
  <p>EDIT: Sorry for the incomplete information, this is the exception:</p>
  
  <pre><code>CRITICAL - Symfony\Component\Form\Exception\UnexpectedTypeException: 
  Expected argument of type "object or array", "string" given (uncaught exception) at
   /var/www/MyBusiness0_1/vendor/symfony/symfony/src/Symfony/Component/Form/Util/PropertyPath.php line 342 
  
  
  /var/www/MyBusiness0_1/vendor/symfony/symfony/src/Symfony/Component/Form/Util/PropertyPath.php at line 342   
      for ($i = 0; $i &lt;= $lastIndex; ++$i) {
          if (!is_object($objectOrArray) &amp;&amp; !is_array($objectOrArray)) {
              throw new UnexpectedTypeException($objectOrArray, 'object or array');
          }
          $property = $this-&gt;elements[$i];
  </code></pre>