title: Doctrine: Value of mappedBy ignored in OneToMany Association
body: <p>I set up a simple User->Comment association in Doctrine, simple OneToMany (One User can write many Comments).</p>
  
  <p>All works find, but i found a strange behavior of Doctrine. First some Code:</p>
  
  <p><strong>User Entity</strong></p>
  
  <pre><code>/**
   * @ORM\Entity
   * @ORM\Table(name="users")
   */
  class User extends EntityAbstract {
      /**
       * @var int
       * @ORM\Id
       * @ORM\Column(name="id", type="integer")
       * @ORM\GeneratedValue
       */
      protected $_id;
  
      /**
       * @var \Doctrine\Common\Collections\Collection
       * @ORM\OneToMany(targetEntity="Comment", mappedBy="TodayIsASunnyDay")
       *                                                  ^^^^^^^^^^^^^^^^ WTF
       */
      protected $_commentsAuthored;
  
      public function __construct() {
          $this-&gt;_commentsAuthored = 
              new \Doctrine\Common\Collections\ArrayCollection();
      }
  }
  </code></pre>
  
  <p><strong>Comment Entity</strong></p>
  
  <pre><code>/**
   * @ORM\Entity
   * @ORM\Table(name="comments")
   */
  class Comment extends EntityAbstract {
      /**
       * @var int
       * @ORM\Id
       * @ORM\Column(name="id", type="integer")
       * @ORM\GeneratedValue
       */
      protected $_id;
  
      /**
       * @var User
       * @ORM\ManyToOne(targetEntity="User")
       * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
       *
       */
      protected $_author;
  }
  </code></pre>
  
  <p>In User, doctrine wants a <em>mappedBy</em> Attribute since its the inversed Side of the association. But as it seems, I can write <strong>anything I want as value</strong>. (the right value would be "user_id" i think?)</p>
  
  <p>So, when is this value ever used or checked?</p>