how to get document id Firestore

Question

I am new to Firestore. I am stuck at getting the document ID from Firestore. Here I create the user with name and age parameters. When I click Submit, it submits to Cloud Firestore. Everything is Ok up to now.

class _MyHomePageState extends State<MyHomePage> {
  late final TextEditingController name = TextEditingController();
  late final TextEditingController age = TextEditingController();
  final DBHelper _helper = DBHelper();
  late final User _user=User();

  @override
  Widget build(BuildContext context) {

    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
             TextField(
              controller: name,
              decoration: const InputDecoration(hintText: "Name"),
            ),
             TextField(
              controller: age,
              decoration: const InputDecoration(hintText: "Age"),
            ),
            ElevatedButton(onPressed:(){
              _user.setAge(age.text);
              _user.setName(name.text);
              print("Name: ${_user.name}");
                _helper.addUser(_user);
                Navigator.push(context, MaterialPageRoute(builder: (builder)=>MainPage( ,))); //here i need to push the document id
            }, child: const Text("Submit"))
          ],
        ),
      ),
    );
  }

This is my custom User model

class User{
  late final String userId; 
  final  String name;
  final  String age;
  User({required this.name, required this.age});


}

This is my service class for Firestore methods

class DBHelper{
 // final User user = User();
  final CollectionReference _reference= FirebaseFirestore.instance.collection("users"); // database path

  //Adding user to Firestore
  Future<void> addUser(User user){
  return _reference.add({
    'name':user.name,
    'age':user.age,
  }).then((value) => print("User added"))

      .catchError((onError)=> print("Failed to add the user: $onError"));
}
} 

Here is what I want: When I click on the submit button in the previous page. It will navigate to the following page. In this page, I want to show the "name" and "age" information. The only thing that is missing is the documentId

    return FutureBuilder<DocumentSnapshot>(
        future: _ref.doc(documentId).get(), //which document doesn't know
        builder:
            (BuildContext context, AsyncSnapshot<DocumentSnapshot> snapshot) {
          if (snapshot.hasError) {
            return const Center(
                child: Text(
              "Bir hata oluştu.",
              textScaleFactor: 3,
            ));
          } else if (snapshot.hasData && !snapshot.data!.exists) {
            return const Center(
                child: Text(
              "Döküman yok",
              textScaleFactor: 3,
            ));
          }
          if (snapshot.connectionState == ConnectionState.done) {
            Map<String, dynamic> data =
                snapshot.data!.data() as Map<String, dynamic>;
            return Scaffold(
              body: Center(
                  child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Text(
                    "Name: ${data['name']}",
                    textScaleFactor: 3,
                  ),
                  Text(
                    "Age:${data['age']}",
                    textScaleFactor: 3,
                  ),
                ],
              )),
            );
          } else {
            return const Center(child: CircularProgressIndicator());
          }
        });

Answer 1

you can get the id on the metodh you used to submit infos

instead of return void,return a String(the id) then go to the next screen like that:

  Future<String> addUser(User user)async {
  return await _reference.add({
    'name':user.name,
    'age':user.age,
  }).then((value) => var documentId=value.id)

and the button function should be:

 ElevatedButton(onPressed:(){
              _user.setAge(age.text);
              _user.setName(name.text);
              print("Name: ${_user.name}");
                var id=await _helper.addUser(_user);
               Navigator.push(Get.context!, MaterialPageRoute(builder:(builder)=>MainPage(documentI: id))); 
            }, child: const Text("Submit"))
          ],
        ),