Doctrine2 Inheritance Mapping - Wrong ID used - entity-framework

Question

i'm havin a little problem at the moment.
There's an parent Entity Comment with CommentType and CommentId and some other fields.
Also there are some differently CommentEntites (like CommentText or CommentPhoto) which extends the parent Entity.
The expected behaviour should be, that Doctrine loads Data with ContentId from Entity CommentText or CommentImage (depending on CommentType).
But at the moment it's loaded with the ID from the Parent Entity.
The Parent Entity:
use Doctrine\ORM\Mapping as ORM;
/**
* Comment
*
* #ORM\Table(name="comments")
* #ORM\Entity
* #ORM\InheritanceType("JOINED")
* #ORM\DiscriminatorColumn(name="comment_type", type="string")
* #ORM\DiscriminatorMap( {"txt" = "CommentTxt", "img" = "CommentImg"} )
*/
abstract class Comment
{
/**
* #var integer
*
* #ORM\Column(name="content_id", type="integer", nullable=true)
*/
private $contentId;
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="IDENTITY")
*/
private $id;
//and so on
One of the Children Entities:
<?php
namespace AppBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
/**
* CommentTxt
*
* #ORM\Table(name="comment_text")
* #ORM\Entity
*/
class CommentText extends Comment
{
/**
* #var string
*
* #ORM\Column(name="content", type="text", length=65535, nullable=false)
*/
private $content;
/**
* #var \DateTime
*
* #ORM\Column(name="created_at", type="datetime", nullable=false)
*/
private $createdAt;
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="IDENTITY")
*/
private $id;
//and some more
How can i change this behaviour?
Greets