flutter - firebase realtime database orderBy not equal to

Question

In my chat list page, number of unread messages is displayed for each chat. And in the bottom navigation bar I want to show total unread messages(sum of all unread in each chat) as a badge next to bottom navigation icon.

First I have to look up 'userChat' to get the list of chatKey, then I should iterate through each 'messages/{chatKey}' to count number of unread.

Problem is that I have to raise the counter only if senderID is NOT the user himself. But there is no orderBy('senderID').notEqualTo(_uid). How can I select items NOT MATCHING certain value?

My code is shown below.

Stream<int> numberOfTotalUnread() {
    int unread = 0;

    return _userChatDB.child(_uid).onValue.map((event) {
      unread = 0;
      if(event.snapshot.value != null) {
        for (var element in event.snapshot.children) {
          print(element.key);

          // this is the problem part          
          _messageDB.child(element.key!).orderByChild('senderID').notEqualTo(_uid).onValue.map((ev) {

            if(ev.snapshot.value != null) {
              print('??');
              for (var el in ev.snapshot.children) {
                print(el.key);
                if(Map<String, dynamic>.from(el.value as dynamic)['read'] != null) unread += 1;
              }
            }
          });
        }
      }
      print(unread);
      return unread;
    });
  }