Modify the Resolver

func (r *mutationResolver) Register(ctx context.Context, input model.NewUser) (string, error) {
    // perform an user registration
    var token string = r.userService.Register(input)
    // if JWT token is empty, return an error
    if token == "" {
        return "", errors.New("registration failed")
    }
    // return JWT token
    return token, nil
}

func (r *mutationResolver) Login(ctx context.Context, input model.LoginInput) (string, error) {
    // perform a login
    var token string = r.userService.Login(input)

    // if JWT token is empty, return an error
    if token == "" {
        return "", errors.New("login failed, invalid email or password")
    }

    // return JWT token
    return token, nil
}

func (r *mutationResolver) NewBlog(ctx context.Context, input model.NewBlog) (*model.Blog, error) {
    // get the user from the middleware
    user := middleware.ForContext(ctx)
    // if the user does not exist, return an error
    if user == nil {
        return &model.Blog{}, errors.New("access denied")
    }

    // create a new blog
    blog, err := r.blogService.CreateBlog(input, *user)

    // if blog creation failed, return an error
    if err != nil {
        return nil, err
    }

    // return the recently created blog
    return blog, nil
}