Problem with api-plateform when using 2 level of inheritance with doctrine entity - symfony

Question

I have a problem in a 2 level of inheritance structure with api-platfom but maybe with doctrine too
I have two question about this code :
doctrine is well generating a discr column in the MyParent class, but not in MyChild class for some reason... I don't think it is normal as how to know that an instance is MyChildChild if there is no discr column in MyChild ?
with api-platform, I have some errors as soon as I have 2 levels of inheritance. As soon as I remove one of the "extends MyChild" or "extends MyParent" api-platform behaviour is ok but loose the fields of the removed class (normal)
Is there something I'm doing wrong ? I have already use multiple level of inheritance with doctrine and I don't remember that I had this behaviour (only one discr column for 2 classes). And for api-platform, I'm just discovering so maybe I missed something
Thank you in advance
RV
Here is my code
// src/Entity/MyParent.php
namespace App\Entity;
use ApiPlatform\Core\Annotation\ApiResource;
use App\Repository\MyParentRepository;
use Doctrine\ORM\Mapping as ORM;
use App\Entity\MyChild;
/**
* #ApiResource()
* #ORM\Entity(repositoryClass=MyParentRepository::class)
* #ORM\InheritanceType("JOINED")
* #ORM\DiscriminatorColumn(name="discr", type="string")
* #ORM\DiscriminatorMap({"child"="MyChild"})
*/
class MyParent
{
/**
* #ORM\Id
* #ORM\GeneratedValue
* #ORM\Column(type="integer")
*/
private $id;
/**
* #ORM\Column(type="string", length=255)
*/
private $parentAttribute;
public function getId(): ?int
{
return $this->id;
}
public function getParentAttribute(): ?string
{
return $this->parentAttribute;
}
public function setParentAttribute(string $parentAttribute): self
{
$this->parentAttribute = $parentAttribute;
return $this;
}
}
namespace App\Entity;
use ApiPlatform\Core\Annotation\ApiResource;
use App\Repository\MyChildRepository;
use Doctrine\ORM\Mapping as ORM;
use App\Entity\MyChildChild;
use App\Entity\MyParent;
/**
* #ApiResource()
* #ORM\Entity(repositoryClass=MyChildRepository::class)
* #ORM\InheritanceType("JOINED")
* #ORM\DiscriminatorColumn(name="discr", type="string")
* #ORM\DiscriminatorMap({"childchild"="MyChildChild"})
*/
class MyChild extends MyParent
{
/**
* #ORM\Id
* #ORM\GeneratedValue
* #ORM\Column(type="integer")
*/
private $id;
/**
* #ORM\Column(type="string", length=255)
*/
private $childAttribute;
public function getId(): ?int
{
return $this->id;
}
public function getChildAttribute(): ?string
{
return $this->childAttribute;
}
public function setChildAttribute(string $childAttribute): self
{
$this->childAttribute = $childAttribute;
return $this;
}
}
<?php
namespace App\Entity;
use ApiPlatform\Core\Annotation\ApiResource;
use App\Repository\MyChildChildRepository;
use Doctrine\ORM\Mapping as ORM;
use App\Entity\MyChild;
/**
* #ApiResource()
* #ORM\Entity(repositoryClass=MyChildChildRepository::class)
*/
class MyChildChild extends MyChild
{
/**
* #ORM\Column(type="string", length=255)
*/
private $MyChildChildAttribute;
public function getMyChildChildAttribute(): ?string
{
return $this->MyChildChildAttribute;
}
public function setMyChildChildAttribute(string $MyChildChildAttribute): self
{
$this->MyChildChildAttribute = $MyChildChildAttribute;
return $this;
}
}