title: Symfony-doctrine: need to set to null a column when i delete his target entity
body: <p>I have a class Zone containing this:</p>
  
  <pre><code>/**
   * @ORM\OneToMany(targetEntity="Planification", mappedBy="zone")
   */
  protected $planifications;
  </code></pre>
  
  <p>and a class Planification with this:</p>
  
  <pre><code>/**
   * @ORM\ManyToOne(targetEntity="Zone", inversedBy="planifications")
   * @ORM\JoinColumn(name="zone_id", referencedColumnName="id", nullable=true, onDelete="SET NULL")
   * @Expose
   */
   protected $zone;
  </code></pre>
  
  <p>But when I go to see in database, zone_id isn't set at null when I delete a Zone.
  And I have any error code. I only see the zone was deleted, that's all.</p>
  
  <p>But if I configure in another way : "when i delete a zone, i delete the planification", it works :</p>
  
  <p>In zone class : </p>
  
  <pre><code>/**
   * @ORM\OneToMany(targetEntity="Planification", mappedBy="zone", orphanRemoval=true, cascade={"remove"})
   */
  protected $planifications;
  </code></pre>
  
  <p>In planification class : </p>
  
  <pre><code>/**
   * @ORM\ManyToOne(targetEntity="Zone", inversedBy="planifications")
   * @ORM\JoinColumn(name="zone_id", referencedColumnName="id", nullable=true)
   * @Expose
   */
   protected $zone;
  </code></pre>
  
  <p>But I don't need to delete the planification when a zone is deleted. I need to set zone_id to null. </p>
  
  <p>Any idea ? </p>