protected function determineModel()
    {
        // Retrieve all of the configured authentication providers that
        // use the LDAP driver and have a configured model.
        $providers = array_where(Config::get('auth.providers'), function ($value, $key) {
            return $value['driver'] == 'ldap' && array_key_exists('model', $value);
        });

        // Pull the first driver and return a new model instance.
        if ($ldap = reset($providers)) {
            return $ldap['model'];
        }

        throw new UnexpectedValueException(
            'Unable to retrieve LDAP authentication driver. Did you forget to configure it?'
        );
    }