public function persistClientToken($clientId, $accessToken, $expiration, $role)
    {
        /** @var \Illuminate\Database\Query\Builder $table */
        $table = app('db')->connection()->table('clients');

        // If the record doesn't already exist, add it.
        if (! $table->where(['client_id' => $clientId])->exists()) {
            $table->insert(['client_id' => $clientId]);
        }

        // Update record with the new access token & expiration.
        $table->where(['client_id' => $clientId])->update([
            'access_token' => $accessToken,
            'access_token_expiration' => $expiration,
        ]);
    }