How to use string or variables to fetch data from firebase realtime database using OrderByChild in flutter

Question

I am trying to fetch all the conferences that contain current user's uid. I am trying to accomplish it by using OrderByChild($string).(firebaseauth.currentuser.uid) as show below in my codes. In my codes, i fetched the current users specific key in string and then used it to fetch all the data that contain that key.

String MyAssigningID = "...";
Future<void> _fetchUserASSIGNINGIDFromDatabase() async {
DatabaseReference reference = FirebaseDatabase.instance.reference().child("User").child(widget.GroupID).child("Group-Members").child(auth.currentUser.uid);
final DataSnapshot data = await reference.once();
setState(() {
  MyAssigningID = data.value['AssigningID'].toString();
});
}

GetGroupFunction() {
DatabaseReference referenceData = FirebaseDatabase.instance.reference().child("User").child(widget.GroupID).child("Conferences");
referenceData.orderByChild(MyAssigningID).equalTo(auth.currentUser.uid).once().then((DataSnapshot dataSnapshot) {
  Conferencelists.clear();
  var keys = dataSnapshot.value.keys;
  var values = dataSnapshot.value;

  for (var key in keys) {
    Conferencemodals conferencemodals = new Conferencemodals
      (
        values [key]['Title'],
        values [key]['Description'],
        values [key]['StartingTime'],
        values [key]['EndingTime'],
        values [key]['Duration'],
        values [key]['ConferenceID'],
        values [key]['SchoolUpin'],
        values [key]['CreatedDate'],
        values [key]['Published'],
        values [key]['ConferenceDay'],
        values [key]['ConferenceHost'],
    );
    Conferencelists.add(conferencemodals);
  }
  setState(() {
    TotalConferences = Conferencelists.length.toString();
  });
});
}