public function findRelatedModel($modelName, $field, $id = NULL) {
    $relatedModel = $this->getInstance($modelName);

    if ($this->isNew() && $id === NULL) {
      throw new \Exception('No id for related');
    }

    if ($id === NULL) {
      $id = $this->getField($field);
    }

    $result = $relatedModel->findById($id);

    return $result;
  }