title: Can be Doctrine make relationship through abstract class
body: <p>I used ZF2, Doctrine </p>
  
  <p>I have several Entity which extends base abstract class.
  One of fields in abstract class have relation with other Entity - message
  My Entity:</p>
  
  <pre><code>class AbstractChat
  {
      ............
  
      /** ONE-TO-MANY BIDIRECTIONAL, INVERSE SIDE
       * @ORM\OneToMany(targetEntity="Chat\Entity\Message", mappedBy="chat")
       */
      protected $messages;
  }
  
  class Chat extends AbstractChat
  {
  
  }
  
  class ChatBuilding extends AbstractChat
  {
  
  }
  
  class Message
  {
  
      /**
       * @var \Chat\Entity\Chat
       *
       * @ORM\ManyToOne(targetEntity="Chat\Entity\AbstractChat", inversedBy="messages")
       * @ORM\JoinColumns({
       *   @ORM\JoinColumn(name="chat_id", referencedColumnName="id")
       * })
       */
      private $chat;
  }
  </code></pre>
  
  <p>How can I state that the relation is with all entities which extend <code>AbstractChat</code> class?</p>
  
  <p>This mean does not work:</p>
  
  <pre><code> @ORM\ManyToOne(targetEntity="Chat\Entity\AbstractChat", inversedBy="messages")
  </code></pre>