title: Symfony 3 / Doctrine - Get changes to associations in entity change set
body: <p>So I already know that I can get changes to a specific entity in the <code>preUpdate</code> lifecycle event: </p>
  
  <pre><code>/**
   * Captures pre-update events.
   * @param PreUpdateEventArgs $args
   */
   public function preUpdate(PreUpdateEventArgs $args)
   {
       $entity = $args-&gt;getEntity();
  
  
       if ($entity instanceof ParentEntity) {
              $changes = $args-&gt;getEntityChangeSet();
       }
   }
  </code></pre>
  
  <p>However, is there a way to also get changes for any associated Entities? For example, say <code>ParentEntity</code> has a relationship setup like so:</p>
  
  <pre><code>/**
   * @ORM\OneToMany(targetEntity="ChildEntity", mappedBy="parentEntity", cascade={"persist", "remove"})
   */
   private $childEntities;
  </code></pre>
  
  <p>And <code>ChildEntity</code> also has:</p>
  
  <pre><code>/**
   * @ORM\OneToMany(targetEntity="GrandChildEntity", mappedBy="childEntity", cascade={"persist", "remove"})
   */
   private $grandChildEntities;
  </code></pre>
  
  <p>Is there a way to get all relevant changes during the <code>preUpdate</code> of <code>ParentEntity</code>? </p>