How to store user_id in session using Ruby Sinatra

Here is my UserService class, once my route get the singup request, it will call UserService.singup to do the signup logic. I want to save user's id in session but it just dosen't work.

It keep showing me "NameError - undefined local variable or method `session' for UserService:Class"

class UserService

    def self.signup(params)
        @user = User.new(params)
        if @user.save
            session[:user_id] = @user.id
            make_json(200, 0, "Signup success!", @user)
        else
            make_json(400, 0, "Signup success!")
        end
    end

end