Flarum\Event\GetApiRelationship::isRelationship PHP Method

    public function isRelationship($serializer, $relationship)
    {
        return $this->serializer instanceof $serializer && $this->relationship === $relationship;
    }

 /**
  * @param GetApiRelationship $event
  * @return \Tobscure\JsonApi\Relationship|null
  */
 public function getApiRelationship(GetApiRelationship $event)
 {
     if ($event->isRelationship(PostBasicSerializer::class, 'mentionedBy')) {
         return $event->serializer->hasMany($event->model, PostBasicSerializer::class, 'mentionedBy');
     }
     if ($event->isRelationship(PostBasicSerializer::class, 'mentionsPosts')) {
         return $event->serializer->hasMany($event->model, PostBasicSerializer::class, 'mentionsPosts');
     }
     if ($event->isRelationship(PostBasicSerializer::class, 'mentionsUsers')) {
         return $event->serializer->hasMany($event->model, PostBasicSerializer::class, 'mentionsUsers');
     }
 }