Flow EventSourcing: Use Value Objects as Event Payload / ReadModel primary keys

ExampleEvent.php

<?php
namespace Some\Package

use Neos\EventSourcing\Event\EventInterface;

final class ExampleEvent implements EventInterface
{
    /**
     * @var UserId
     */
    private $userId;

    public function __construct(UserId $userId)
    {
        $this->userId = $userId;
    }

    public function getUserId(): UserId
    {
        return $this->userId;
    }
}

Settings.yaml

Neos:
  Flow:
    persistence:
      doctrine:
        dbal:
          mappingTypes:
            'user_id':
              dbType: string
              className: 'Some\Package\UserIdDbalType'

User.php

<?php
namespace Some\Package

use Doctrine\ORM\Mapping as ORM;
use Neos\EventSourcing\Annotations as ES;
use Neos\Flow\Annotations as Flow;

/**
 * @Flow\Entity
 * @ES\ReadModel
 */
class User
{
    /**
     * @var UserId
     * @ORM\Id
     * @ORM\Column(type="user_id")
     */
    protected $id;

    public function __construct(UserId $id)
    {
        $this->id = $id;
    }

    public function getId(): UserId
    {
        return $this->id;
    }
}

UserId.php

<?php
namespace Some\Package;

use Neos\Flow\Annotations as Flow;

/**
 * @Flow\Proxy(false)
 */
final class UserId implements \JsonSerializable
{
    /**
     * @var string
     */
    private $value;

    public function __construct(string $value)
    {
        $this->value = $value;
    }

    public function jsonSerialize(): string
    {
        return $this->value;
    }

    public function __toString(): string
    {
        return $this->value;
    }
}

UserIdDbalType.php

<?php
namespace Some\Package;

use Doctrine\DBAL\Platforms\AbstractPlatform;
use Doctrine\DBAL\Types\Type;

final class UserIdDbalType extends Type
{
    public function getName()
    {
        return 'user_id';
    }

    public function getSQLDeclaration(array $fieldDeclaration, AbstractPlatform $platform)
    {
        return $platform->getVarcharTypeDeclarationSQL($fieldDeclaration);
    }

    public function convertToPHPValue($value, AbstractPlatform $platform)
    {
        return (null === $value) ? null : new UserId($value);
    }

    public function getDefaultLength(AbstractPlatform $platform)
    {
        return 40;
    }
}