code: protected function findRelated($record, array $relationship)
      {
          $inverse = $this->getRelation($record, $this->key)->getRelated();
          $related = $this->findToMany($relationship);
  
          $related = collect($related)->filter(function ($model) use ($inverse) {
              return $model instanceof $inverse;
          });
  
          return new Collection($related);
      }
docs: Find the related models for a JSON API relationship object.
  
  We look up the models via the store. These then have to be filtered to
  ensure they are of the correct model type, because this has-many relation
  might be used in a polymorphic has-many JSON API relation.
  
  @todo this is currently inefficient for polymorphic relationships. We
  need to be able to filter the resource identifiers by the expected resource
  type before looking them up via the store.
  
  @param $record
  @param array $relationship
  @return Collection