title: change default identity column name in inherited (joined ) class in symfony
body: <p>I have used a Class Table Inheritance so that my entity "image" inherits from the "media" Entity. 
  this procedure create an auto generated column id in the image table. How can i change this default name "id" to set it "media_id" so that the image column has a column "media_id" instead of  "id" ? 
  here is my code :</p>
  
  <pre><code>namespace App\Entity\OldApp;
  
  use Doctrine\ORM\Mapping as ORM;
  use \App\Entity\OldApp\TemplateContent;
  
  /**
   * @ORM\Entity(repositoryClass="App\Repository\OldApp\MediaRepository")
   * @ORM\InheritanceType("JOINED")
   * @ORM\DiscriminatorColumn(name="type", type="string")
   * @ORM\DiscriminatorMap({"media" = "Media", "image" = "Image"})
   */
  
  class Media
  {
  
      /**
       * @ORM\Id()
       * @ORM\GeneratedValue()
       * @ORM\Column(type="integer",name="CAT_id")
       */
      private $id;
  
      /**
       * @ORM\Column(name="filename", type="string", length=100, nullable=false)
       */
      private $filename;
  
  
      /**
       * @ORM\Column(name="size", type="string", length=30, nullable=false)
       */
      private $size;
  
      /**
       * @ORM\Column(name="date", type="datetime", nullable=false)
       */
      private $date;
  
      /**
       * @ORM\Column(name="high", type="boolean", nullable=false)
       */
      private $high = '0';
  
  
  namespace App\Entity\OldApp;
  
  use Doctrine\ORM\Mapping as ORM;
  
  /**
  * @ORM\Entity(repositoryClass="App\Repository\OldApp\ImageRepository")
  */
  class Image extends Media
  {
  
  
  
  /**
  * @ORM\Column(type="string", length=3)
  */
  private $extension;
  
  
  private static $type='image' ;
  
  /**
  * @ORM\Column(type="string", length=50)
  */
  private $ratio;
  
  
  /**
  * @ORM\Column(type="smallint")
  */
  private $height;
  
  /**
  * @ORM\Column(type="smallint")
  */
  private $width;
  }
  </code></pre>
  
  <p>i've tryed to add <code>@PrimaryKeyJoinColumn(name="person_id")</code> Bellow @ORM\Entity but i ged the following error </p>
  
  <p><strong>Error</strong></p>
  
  <blockquote>
    <p>[Semantical Error] The annotation "@Doctrine\ORM\Mapping\PrimaryKeyJoinColumn" in class App\Entity\OldApp\Image doe
      s not exist, or could not be auto-loaded.</p>
  </blockquote>