public function search(string $model, array $fields) : ?array
  {
    $response = array();
    foreach($fields as $param => $value) {
      $check = $model::where($param, $value)->first();

      if ($check != null) {
        $response = array_merge($response, array($param => "The $param has already been taken."));
      }
    }

    return $response;
  }