title: __toString return name from another entity
body: <p>Let say I have 2 tables in my database : lesson and group. Group can have 1 lesson per year. </p>
  
  <p>Then I will have 2 entities, lesson entity :</p>
  
  <pre><code>&lt;?php
  namespace Sifo\AdminBundle\Entity;
  use Doctrine\ORM\Mapping as ORM;
  
  class Lesson
  {
      /**
      * @var integer
      */
      private $id;
  
  
      /**
      * @var string
      */
      private $name;
  
      /**
      * Set id
      *
      * @param integer $id
      * @return DftGrupMapel
      */
      public function setId($id)
      {
          $this-&gt;id = $id;
          return $this;
      }
  
      /**
       * Get id
       *
       * @return integer 
       */
      public function getId()
      {
          return $this-&gt;id;
      }
  </code></pre>
  
  <p>and group entity :</p>
  
  <pre><code>&lt;?php
  namespace Sifo\AdminBundle\Entity;
  use Doctrine\ORM\Mapping as ORM;
  
  class Group
  {
      /**
      * @var integer
      */     
      private $id;
  
      /**
       * @var \Sifo\AdminBundle\Entity\MstLesson
       */
      private $idLesson;
  
      /**
      * @var integer
      */
      private $year;
  
      /**
      * Set id
      *
      * @param integer $id
      * @return DftGrup
      */
      public function setId($id)
      {
          $this-&gt;id = $id;
          return $this;
      }
  
      /**
       * Get id
       *
       * @return integer 
       */
      public function getId()
      {
          return $this-&gt;id;
      }
  
      /**
       * Set idLesson
       *
       * @param \Sifo\AdminBundle\Entity\MstLesson $idLesson
       * @return DftGrup
       */
      public function setIdLesson(\Sifo\AdminBundle\Entity\MstLesson $idLesson = null)
      {
          $this-&gt;idLesson = $idLesson;
          return $this;
      }
  
      /**
       * Get idGrup
       *
       * @return \Sifo\AdminBundle\Entity\MstGrup 
       */
      public function getIdLesson()
      {
          return $this-&gt;idLesson;
      }
  
      /**
      * Set year
      *
      * @param integer $year
      * @return DftGrup
      */
      public function setYear($year)
      {
          $this-&gt;year = $year;
          return $this;
      }
  
      /**
       * Get year
       *
       * @return integer 
       */
      public function getYear()
      {
          return $this-&gt;year;
      }
  </code></pre>
  
  <p>The problem is, in <strong>group</strong> entity I don't have available string to show for <code>__toString()</code>. I fell not good for making new <code>name</code> field in <strong>group</strong> entity and save <strong>lesson</strong> <code>name</code> there. </p>
  
  <p><code>lesson.id</code> and <code>group.idLesson</code> is foreign key.</p>
  
  <p>How I can get <code>name</code> field in <strong>lesson</strong> entity to show in <code>__toString()</code> <strong>group</strong> without create new field? </p>