public function validateRefreshToken($refreshToken, $clientId)
    {
        $result = DB::table('oauth_session_refresh_tokens')
                    ->where('refresh_token', $refreshToken)
                    ->where('client_id', $clientId)
                    ->where('refresh_token_expires', '>=', time())
                    ->first();

        return (is_null($result)) ? false : $result->session_access_token_id;
    }