how to get value from map in firebase database flutter - firebase

Question

i have a json structure like this
{
"-My6relBpWvPaY_I4JvN": {
"idUser": "4dca8440-a37d-11ec-9c66-9b8f61be17f0",
"message": "777777"
}
}
and I wanna use fromJson to save to map,
here is my model:
class Message {
final String userId;
final String message;
const Message({
#required this.userId,
#required this.message,
});
static Message fromJson(Map<String, dynamic> json) => Message(
userId: json['idUser'],
message: json['message'],
);
Map<String, dynamic> toJson() => {
'idUser': userId,
'message': message,
};
}
right now I always get null from the help from Frank,
final data = Map<String, dynamic>.from(snapshot.value as Map);
Message message = Message.fromJson(data);
print('message value${message.message}'); // here i got null
please tell me where is wrong.
---------- Update ----------
Hi Frank, firstly thanks for your reply, I will get a stream from firebase RTDB, since the data structure is as displayed above, with the data I got from following method,
final Map<String, dynamic> data = Map<String, dynamic>.from(snapshot.value as Map);
the key from this data will be {"-My6relBpWvPaY_I4JvN"}, and value is
{
"idUser": "4dca8440-a37d-11ec-9c66-9b8f61be17f0",
"message": "777777"
}
so in this case I need to update the method you showed
//**instead** snapshot.children.forEach((msgSnapshot) {
data.values.forEach((msgSnapshot) {
final Map<String, dynamic> messageData = Map<String, dynamic>.from(msgSnapshot as Map);
//i am not sure whether I need to use msgSnapshot.value as Map, since msgSnaphot is already the right Map<String, dynamic> that I can put to fromJson
Message message = Message.fromJson(messageData);
})
Do I understand correctly? thanks for further info!!

Answer 1

I have it setup like this in my case witch works just fine:
static Message model({required Map map}) => Message(
messageID: map[kModelMessengerID],
owner: map[kModelMessageOwner],
type: map[kModelMessageType],
msg: map[kModelMessageMsg],
membersIDs: (map[kModelMessageMembersIDs] as List<dynamic>)
.map((e) => e.toString())
.toList(),
createdAt: map[kModelMessageCreatedAt] == null
? DateTime.now()
: (map[kModelMessageCreatedAt] as Timestamp).toDate(),
);
and with how I pass data in to it:
final ref = FirebaseDatabase.instance.ref();
final snapshot = await ref.child(...).get();
if (snapshot.exists) {
var m = Message.model(map: snapshot.value as Map))
} else {
print('No data available.');
}

Answer 2

The code in your question doesn't show how snapshot is initialized, but my guess is that it contains a list of messages. Even a list of 1 messages is still a list, so you have to handle that in your code by iterating of the children property of the snapshot:
snapshot.children.forEach((msgSnapshot) {
final data = Map<String, dynamic>.from(msgSnapshot.value as Map);
Message message = Message.fromJson(data);
print('message value${message.message}');
})

Answer 3

Your JSON should look like this:
{"userId": "4dca8440-a37d-11ec-9c66-9b8f61be17f0", "message": "777777"}