/**
  * Return a simple lists string.
  *
  * @return null|string
  */
 public function lists($column = null)
 {
     /* @var Relation $value */
     $value = $this->object->getValue();
     $related = $this->object->getRelatedModel();
     /* @var EloquentCollection $relations */
     if ($relations = $value->get()) {
         return implode(', ', $relations->lists($column ?: $related->getTitleName()));
     }
     return null;
 }