A simple Query in flutter/firebase database - firebase

Question

I try to experience Firebase Live database with flutter.
I just would like to get a value in the datasnapshot of the firebase response.
My Firebase
My Code
static Future<User> getUser(String userKey) async {
Completer<User> completer = new Completer<User>();
String accountKey = await Preferences.getAccountKey();
FirebaseDatabase.instance
.reference()
.child("accounts")
.child(accountKey)
.child("users")
.childOrderBy("Group_id")
.equals("54")
.once()
.then((DataSnapshot snapshot) {
var user = new User.fromSnapShot(snapshot.key, snapshot.value);
completer.complete(user);
});
return completer.future;
}
}
class User {
final String key;
String firstName;
Todo.fromJson(this.key, Map data) {
firstname= data['Firstname'];
if (firstname== null) {
firstname= '';
}
}
}
I got Null value for firstname.
I guess I should navigate to the child of snapshot.value. But impossible to manage with foreach, or Map(), ...
Kind regards, Jerome

Answer 1

You are querying with a query and the documentation for Queries (here in JavaScript, but it is valid for all languages), says that "even when there is only a single match for the query, the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result."
I don't know exactly how you should loop, in Flutter/Dart, over the children of the snapshot but you should do something like the following (in JavaScript):
snapshot.forEach(function(childSnapshot) {
var childKey = childSnapshot.key;
var childData = childSnapshot.val();
// ...
});
and assuming that your query returns only one record ("one single match"), use the child snapshot when you do
var user = new User.fromSnapShot(childSnapshot.key, childSnapshot.value);

Answer 2

This will give you Users in reusable dialog. There might be slight disservice to yourself if you don't use stream and stream-builders, the solution below is a one time fetch of the users' collection on FirebaseDB.
class User {
String firstName, groupID, lastName, pictureURL, userID;
User({this.firstName, this.groupID, this.lastName, this.pictureURL, this.userID});
factory User.fromJSON(Map<dynamic, dynamic> user) => User(firstName: user["Firstname"], groupID: user["Group_id"], lastName: user["Lastname"], pictureURL: user["Picturelink"], userID: user["User_id"]);
}
Future<List<User>> users = Firestore.instance.collection("users").snapshots().asyncMap((users) {
return users.documents.map((user) => User.fromJSON(user.data)).toList();
}).single;