title: problem understanding relation mapping in doctrine 2
body: <p>I read official documentation and tons of threads but still do not find solution for my situation. My case is very basic. I have 2 entities: comments and keywords for them. One comment can have many keywords but each keyword is only for one comment. Keywords are not unique in keyword table. So i decided this is one-to-many relation. Tables structure are simply like follows:</p>
  
  <p><strong><em>keywords</em></strong></p>
  
  <pre><code>id          int(11)
  comment_id  int(11)
  text        varchar(30)
  </code></pre>
  
  <p><strong><em>comments</em></strong></p>
  
  <pre><code>id      int(11)
  text    text
  </code></pre>
  
  <p>here is how i mapped them:</p>
  
  <pre><code>
  /**
   *  @Entity
   *  @Table(name="comments")
   **/
  class Comments
  {
      /** @Id @Column(type="integer") */
      private $id;
      /** @Column(type="text") */
      private $text;
  
      /**
       * @OneToMany(targetEntity="keywords", mappedBy="comment_id")
       */
      private $keywords;
  
      public function getText(){return $this->text;}
      public function getId(){return $this->id;}
      public function getKeywords(){return $this->keywords;}
  }
  /**
   *  @Entity
   *  @Table(name="keywords")
   */
  
  class Keywords
  {
      /** @Id @Column(type="integer") */
      private $id;
  
      private $text;
  
      public function getText(){return $this->text;}
      public function getId(){return $this->id;}
  }
  </code>
  </pre>
  
  <p>and how using it is like this:</p>
  
  <pre><code>
  $comments = $this->em->getRepository('comments' )->findAll();
  foreach($comments as $comment){
      foreach($comment->getKeywords() as $keyword){
          $keyword->getText();
      }
  }</code></pre>
  
  <p>and got this errors:
  <code><pre>
  Notice: Undefined index: comment_id in C:\web_includes\doctrine\ORM\Persisters\BasicEntityPersister.php on line 1096
  Notice: Trying to get property of non-object in C:\web_includes\doctrine\ORM\Persisters\BasicEntityPersister.php on line 1098
  Warning: Invalid argument supplied for foreach() in C:\web_includes\doctrine\ORM\Persisters\BasicEntityPersister.php on line 1098
  Notice: Undefined index: comment_id in C:\web_includes\doctrine\ORM\PersistentCollection.php on line 168
  Fatal error: Call to a member function setValue() on a non-object in C:\web_includes\doctrine\ORM\PersistentCollection.php on line 169
  </pre>
  </code>
  What is wrong? where is should define comment_id? Is my mapping correct? i really stuck and need help so please, any advice are very welcome.</p>