def auth(self, request):
        """
            let's auth the user to the Service
            :param request: request object
            :return: callback url
            :rtype: string that contains the url to redirect after auth

        """
        service = UserService.objects.get(user=request.user, name='ServiceWallabag')
        callback_url = '%s://%s%s' % (request.scheme, request.get_host(), reverse('wallabag_callback'))
        params = {'username': service.username,
                  'password': service.password,
                  'client_id': service.client_id,
                  'client_secret': service.client_secret}
        access_token = Wall.get_token(host=service.host, **params)
        request.session['oauth_token'] = access_token
        return callback_url