Entity to it's ID serialization handler

Assessment.php

<?php

namespace MyVendor\MyBundle\Model;

use MyVendor\MyBundle\Entity\User;
use JMS\Serializer\Annotation as JMS;

class Assessment
{
    ...

    /**
     * @JMS\Type("EntityId<'MyVendorMyBundle:User'>")
     * @var User
     */
    private $user;
    
    ...
  }

EntityIdSerializeHandler.php

<?php

namespace MyVendor\MyBundle\Handler;

use Doctrine\ORM\EntityManager;
use JMS\Serializer\Context;
use JMS\Serializer\GraphNavigator;
use JMS\Serializer\Handler\SubscribingHandlerInterface;
use JMS\Serializer\JsonSerializationVisitor;
use JMS\Serializer\JsonDeserializationVisitor;

class EntityIdSerializeHandler implements SubscribingHandlerInterface
{
    /** @var EntityManager */
    private $em;

    /**
     * @param EntityManager $em
     */
    public function __construct(EntityManager $em)
    {
        $this->em = $em;
    }

    /**
     * Return format:
     *
     *      array(
     *          array(
     *              'direction' => GraphNavigator::DIRECTION_SERIALIZATION,
     *              'format' => 'json',
     *              'type' => 'DateTime',
     *              'method' => 'serializeDateTimeToJson',
     *          ),
     *      )
     *
     * The direction and method keys can be omitted.
     *
     * @return array
     */
    public static function getSubscribingMethods()
    {
        return array(
            array(
                'direction' => GraphNavigator::DIRECTION_SERIALIZATION,
                'format' => 'json',
                'type' => 'EntityId',
                'method' => 'serializeEntityToId',
            ),
            array(
                'direction' => GraphNavigator::DIRECTION_DESERIALIZATION,
                'format' => 'json',
                'type' => 'EntityId',
                'method' => 'deserializeIdToEntity',
            )
        );
    }

    /**
     * @param JsonSerializationVisitor $visitor
     * @param object|null $entity
     * @param array $type
     * @param Context $context
     * @return mixed
     * @throws \Exception
     */
    public function serializeEntityToId(JsonSerializationVisitor $visitor, $entity, array $type, Context $context)
    {
        $entityName = $this->getEntityName($type);

        if ($entity === null) {
            return null;
        }

        if (!is_object($entity)) {
            throw new \Exception(sprintf('Value of @JMS\Type("EntityId<\'%s\'>") must me object of null, %s given.', $entityName, gettype($entity)));
        }

        $identifier = $this->getEntityIdentifier($type);
        $getter = 'get' . ucfirst($identifier);
        $id = $entity->$getter();

        return $id;
    }

    /**
     * @param JsonDeserializationVisitor $visitor
     * @param int|null $id
     * @param array $type
     * @param Context $context
     * @return bool|\Doctrine\Common\Proxy\Proxy|null|object
     * @throws \Doctrine\ORM\ORMException
     */
    public function deserializeIdToEntity(JsonDeserializationVisitor $visitor, $id = null, array $type, Context $context)
    {
        if ($id === null) {
            return null;
        }

        $entityName = $this->getEntityName($type);
        if ($entity = $this->em->getReference($entityName, $id)) {
            return $entity;
        }

        return null;
    }

    /**
     * @param array $type
     * @return string
     * @throws \Exception
     */
    private function getEntityName(array $type)
    {
        if (!(is_array($type['params']) && count($type['params']) && strlen($type['params'][0]))) {
            throw new \Exception('You must specify entityName in @JMS\Type("EntityId<\'entity:name\'>") annotation.');
        }
        $entityName = $type['params'][0];

        return $entityName;
    }

    /**
     * @param array $type
     * @return string
     * @throws \Exception
     */
    private function getEntityIdentifier(array $type)
    {
        $entityName = $this->getEntityName($type);

        if (!$classMetadata = $this->em->getClassMetadata($entityName)) {
            throw new \Exception(sprintf('Can\'t find metadata for class %s', $entityName));
        }

        $identifiers = $classMetadata->getIdentifier();
        if (count($identifiers) != 1) {
            throw new \Exception(sprintf('@JMS\Type("EntityId<>") supports entities with only one identifier, %s contains %s identifier(s).', $entityName, count($identifiers)));
        }

        $identifier = $identifiers[0];

        return $identifier;
    }
}

services.yml

serializer_handler.entity_id:
        ...
        
        class: MyVendor\MyBundle\Handler\EntityIdSerializeHandler
        tags:
            - { name: jms_serializer.subscribing_handler }
        arguments: [@doctrine.orm.entity_manager]
        
        ...