SWIFT-从随机创建的文档ID获取字段(Firestore)

在我的应用程序中，用户可以注册，通过这样做，我还可以保存firstname, lastname和uid.

FiRest-Database是这样的：

https://i.stack.imgur.com/TC24vl.png

我想访问用户lastname但我不知道怎么做，作为Document-ID在注册过程中随机创建。

Auth.auth().createUser(withEmail: email, password: password) { (result, err) in

            //check for errors
            if let err = err {
                self.view.endEditing(true)
                self.showError(err.localizedDescription)

            }else {

                //user was created successfully; store first and last name
                let db = Firestore.firestore()

                db.collection("users").addDocument(data: ["firstname":firstName, "lastname": lastName, "uid": result!.user.uid]) { (error) in

                    if error != nil {
                        self.showError("Error saving user data")
                    }
                }

                //transition to home
                self.transitionToHome()
            }