class RelationshipRouteAccessCheck

final class RelationshipRouteAccessCheck implements AccessInterface {
    
    /**
     * The route requirement key for this access check.
     *
     * @var string
     */
    const ROUTE_REQUIREMENT_KEY = '_jsonapi_relationship_route_access';
    
    /**
     * The JSON:API entity access checker.
     *
     * @var \Drupal\jsonapi\Access\EntityAccessChecker
     */
    protected $entityAccessChecker;
    
    /**
     * RelationshipRouteAccessCheck constructor.
     *
     * @param \Drupal\jsonapi\Access\EntityAccessChecker $entity_access_checker
     *   The JSON:API entity access checker.
     */
    public function __construct(EntityAccessChecker $entity_access_checker) {
        $this->entityAccessChecker = $entity_access_checker;
    }
    
    /**
     * Checks access to the relationship field on the given route.
     *
     * @param \Symfony\Component\Routing\Route $route
     *   The route to check against.
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The route match.
     * @param \Drupal\Core\Session\AccountInterface $account
     *   The currently logged in account.
     *
     * @return \Drupal\Core\Access\AccessResultInterface
     *   The access result.
     */
    public function access(Route $route, RouteMatchInterface $route_match, AccountInterface $account = NULL) {
        [
            $relationship_field_name,
            $field_operation,
        ] = explode('.', $route->getRequirement(static::ROUTE_REQUIREMENT_KEY));
        assert(in_array($field_operation, [
            'view',
            'edit',
        ], TRUE));
        $entity_operation = $field_operation === 'view' ? 'view' : 'update';
        if ($resource_type = $route_match->getParameter(Routes::RESOURCE_TYPE_KEY)) {
            assert($resource_type instanceof ResourceType);
            $entity = $route_match->getParameter('entity');
            $internal_name = $resource_type->getInternalName($relationship_field_name);
            if ($entity instanceof FieldableEntityInterface && $entity->hasField($internal_name)) {
                $entity_access = $this->entityAccessChecker
                    ->checkEntityAccess($entity, $entity_operation, $account);
                $field_access = $entity->get($internal_name)
                    ->access($field_operation, $account, TRUE);
                // Ensure that access is respected for different entity revisions.
                $access_result = $entity_access->andIf($field_access);
                if (!$access_result->isAllowed()) {
                    $reason = "The current user is not allowed to {$field_operation} this relationship.";
                    $access_reason = $access_result instanceof AccessResultReasonInterface ? $access_result->getReason() : NULL;
                    $detailed_reason = empty($access_reason) ? $reason : $reason . " {$access_reason}";
                    $access_result->setReason($detailed_reason);
                }
                return $access_result;
            }
        }
        return AccessResult::neutral();
    }

}