core: Wrong attributes serialized when embedding multiple different inherited entities

When serializing an related entity which hast some kind of entity with inheritance embedded (doesn’t actually matter if it’s single table or joined). The properties of the parent class are serialized, and ONLY the properties of one of the child classes. But for all entites, regardless of their type.

This is a sample setup:

/**
 * @ApiResource()
 * @ORM\Entity
 * @ORM\InheritanceType(value="JOINED")
 */
class ParentClass
{
    /**
     * @var int The entity Id
     *
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     * @Groups({"get"})
     */
    protected $id;

    /**
     * @var string Something else
     *
     * @ORM\Column
     * @Assert\NotBlank
     * @Groups({"get"})
     */
    protected $baz = '';
}

/**
 * @ApiResource
 * @ORM\Entity
 */
class BarChild extends ParentClass
{
    /**
     * @var int The entity Id
     *
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    protected $id;

    /**
     * @var string Something else
     *
     * @ORM\Column
     * @Assert\NotBlank
     * @Groups({"get"})
     */
    private $bar = '';
}

/**
 * @ApiResource
 * @ORM\Entity
 */
class FooChild extends ParentClass
{
    /**
     * @var int The entity Id
     *
     * @ORM\Id
     * @ORM\GeneratedValue
     * @ORM\Column(type="integer")
     */
    protected $id;

    /**
     * @var string Something else
     *
     * @ORM\Column
     * @Assert\NotBlank
     * @Groups({"get"})
     */
    private $foo = '';

/**
 * @ApiResource(attributes={"normalization_context"={"groups"={"get"}}})
 * @ORM\Entity()
 */
class RelatedEntity
{
    /**
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     * @ORM\Column(type="integer")
     * @Groups({"get"})
     */
    private $id;

    /**
     * @ORM\Column(type="string")
     * @Groups({"get"})
     */
    private $attribute;

    /**
     * @ORM\OneToMany(targetEntity="AppBundle\Entity\ParentClass", mappedBy="relatedEntity")
     * @Groups({"get"})
     */
    private $relatedEntites;
}

When retrieving a RelatedEntity which has a relation to one FooChild and one BarChild the result looks like this:

{
  "@context": "\/app_dev.php\/contexts\/RelatedEntity",
  "@id": "\/app_dev.php\/related_entities\/1",
  "@type": "RelatedEntity",
  "id": 1,
  "attribute": "test",
  "relatedEntites": [
    {
      "@id": "\/app_dev.php\/foo_children\/1",
      "@type": "FooChild",
      "id": 1,
      "baz": "test2",
      "bar": null
    },
    {
      "@id": "\/app_dev.php\/bar_children\/2",
      "@type": "BarChild",
      "id": 2,
      "baz": "testbaz",
      "bar": ""
    }
  ]
}

But the FooChild Entity should not have the property bar, but foo instead. And in my testing date the value of baron the BarChild entity is actually not an empty string…