Flutter Firebase：如何通过用户名获取用户ID？

Question

I'm actually working on a social app. I want to create a friend system and I'm struggling with the user search. I want the user to put the username of his friend in a TextField widget, query the Cloud Firestore (Firebase) database and get the friend's user profile if it exists.

The problem is, usernames are stored in "Users/USER_ID/profile/username" and I don't know how to query the database for this demand.

Here is the Cloud Firestore "path" i'm talking about: -

A little help would be welcome:)

I tried this but this returns null , probably because it is not searching in the right place.

dynamic sUserFromUsername(String username) async {
  try {
    await FirebaseFirestore.instance
      .collection('Users')
      .where('username', isEqualTo: username)
      .get()
      .then((value) => value);
  } catch (exception) {
    print(exception);
  }
}