code: protected function prepareKeys($models)
      {
          if ($this->key !== null) {
              $keys = [];
              foreach ($models as $model) {
                  if (is_string($this->key)) {
                      $keys[] = $model[$this->key];
                  } else {
                      $keys[] = call_user_func($this->key, $model);
                  }
              }
  
              return $keys;
          }
  
          return array_keys($models);
      }
docs: {@inheritdoc}