json – RealTime Database with Flutter overwrites data instead of adding it to the existing nodes

I am using flutter with Firebase Realtime Database. As shown in the code, each user can create his lists and add tasks to it. However, every time I try to add a new task to the same list it overwrites the existing data instead of appending to it. Can someone please help me out.

This is the ToDo model

class Todo {
  String group;
  String subject;
  bool completed;
  String userId;

  Todo(this.userId, this.group, this.subject, this.completed);

  Todo.fromSnapshot(DataSnapshot snapshot)
      : group = snapshot.value("group"),
        userId = snapshot.value("userId"),
        subject = snapshot.value("subject"),
        completed = snapshot.value("completed");

  toJson() {
    return {
      userId: {
        "lists": (
            "groupname": group,
            "tasks": (
              {"name": subject, "done": completed},

This is the function to add to the database

  addNewTodo(String groupn, String taskn) {
    if (groupn.length > 0 && taskn.length > 0) {
      Todo todo =
          new Todo(widget.userId, groupn.toString(), taskn.toString(), false);

groupn & taskn are the variables inputted by the user.