api-platform write listener: skip creating entity in special cases

CampaignParticipationApiSubscriber.php

<?php

/**
 * Created by PhpStorm.
 * User: Thomas Kekeisen <thomas@kekeisen.it>
 * Date: 13.04.17
 * Time: 11:38
 */

namespace AppBundle\Subscriber;

use ApiPlatform\Core\EventListener\EventPriorities;
use AppBundle\Entity\CampaignParticipation;
use AppBundle\Repository\CampaignParticipationRepository;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\GetResponseForControllerResultEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorage;

class CampaignParticipationApiSubscriber implements EventSubscriberInterface
{
    /**
     * @var $campaignParticipationRepository CampaignParticipationRepository
     */
    protected $campaignParticipationRepository;

    /**
     * @var TokenStorage
     */
    protected $tokenStorage;

    /**
     * CampaignParticipationApiSubscriber constructor.
     * @param CampaignParticipationRepository $campaignParticipationRepository
     * @param TokenStorage $tokenStorage
     */
    public function __construct(
        CampaignParticipationRepository $campaignParticipationRepository,
        TokenStorage $tokenStorage
    )
    {
        $this->campaignParticipationRepository = $campaignParticipationRepository;
        $this->tokenStorage = $tokenStorage;
    }

    /**
     * @return array
     */
    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::VIEW => [['checkForDuplicatedCampaignParticipation', EventPriorities::PRE_WRITE]],
        ];
    }

    /**
     * @param GetResponseForControllerResultEvent $event
     */
    public function checkForDuplicatedCampaignParticipation(GetResponseForControllerResultEvent $event)
    {
        // @formatter:off
        $token                          = $this->tokenStorage->getToken();
        $user                           = $token->getUser();
        /**
         * @var $campaignParticipationToCreate CampaignParticipation
         */
        $campaignParticipationToCreate  = $event->getControllerResult();
        $campaign                       = $campaignParticipationToCreate->getCampaign();
        $existingCampaignParticipations = $this->campaignParticipationRepository->findOpenByUserAndCampaign(
            $user,
            $campaign
        );

        if (count($existingCampaignParticipations) > 0) {
            $existingCampaignParticipation = $existingCampaignParticipations[0];

            // This will return the existing entity instead of creating a new entity
            $event->setControllerResult($existingCampaignParticipation);
        }
    }
}

services.yml

campaign_participation.api_subscriber:
    arguments:
        - '@=service("doctrine.orm.entity_manager").getRepository("AppBundle:CampaignParticipation")'
        - '@security.token_storage'
    class: AppBundle\Subscriber\CampaignParticipationApiSubscriber
    tags:
        - { name: kernel.event_subscriber }