How to properly sort list by another list in Dart

Question

I have two lists, 1 is a list of Map items, and another list which is the order.
I would like to sort the items based on their description attribute and compare them with the order list and have them inserted at the top.

import 'package:collection/collection.dart';

void main() {
  List<String> order = [
    'top european',
    'top usa',
    'top rest of the world'
  ];

  List<Map> items = [
    {'id': 0, 'id2': 5, 'description': 'Top USA'},
    {'id': 2, 'id2': 2, 'description': 'Top A'},
    {'id': 3, 'id2': 0, 'description': 'Top Z'},
    {'id': 6, 'id2': 6, 'description': 'Top Rest of the world'},
    {'id': 4, 'id2': 4, 'description': 'Top C'},
    {'id': 5, 'id2': 1, 'description': 'Top D'},
    {'id': 1, 'id2': 3, 'description': 'Top European'},
  ];
  
  //this works but adds the items at the end
  items.sort((a,b)  {
    return order.indexOf(a['description'].toLowerCase()) - 
      order.indexOf(b['description'].toLowerCase());
  });

  ///Results: print(items);
  // List<Map> items = [
  //   {'id': 2, 'id2': 2, 'description': 'Top A'},
  //   {'id': 3, 'id2': 0, 'description': 'Top Z'},
  //   {'id': 4, 'id2': 4, 'description': 'Top C'},
  //   {'id': 5, 'id2': 1, 'description': 'Top D'},
  //   {'id': 1, 'id2': 3, 'description': 'Top European'},
  //   {'id': 0, 'id2': 5, 'description': 'Top USA'},
  //   {'id': 6, 'id2': 6, 'description': 'Top Rest of the world'},
  // ];
}

SOLUTION: I also tried this approach which is not ideal, but it works.

List <Map> itemsOrder = items
  .where(
    (ele) => order.contains(ele['description'].toString().toLowerCase()))
  .toList();

itemsOrder.sort((a, b) {
  return order.indexOf(a['description'].toLowerCase()) -
    order.indexOf(b['description'].toLowerCase());
});

items.removeWhere(
  (ele) => order.contains(ele['description'].toString().toLowerCase()));

itemsOrder = itemsOrder.reversed.toList();

for (int i = 0; i < itemsOrder.length; i++) {
  items.insert(0, itemsOrder[i]);
}

///Results: print(items);
// List<Map> items = [
//   {'id': 1, 'id2': 3, 'description': 'Top European'},
//   {'id': 0, 'id2': 5, 'description': 'Top USA'},
//   {'id': 6, 'id2': 6, 'description': 'Top Rest of the world'},
//   {'id': 2, 'id2': 2, 'description': 'Top A'},
//   {'id': 3, 'id2': 0, 'description': 'Top Z'},
//   {'id': 4, 'id2': 4, 'description': 'Top C'},
//   {'id': 5, 'id2': 1, 'description': 'Top D'},
// ];

Ideally, I would like to use sortBy or sortByCompare but unfortunately, I cannot find a proper example or get a grasp of how to use it.

Answer 1

The way I would fix this is to find the index of the description in the order list and if it cannot be found, I would use a number that is out of index inside the order list to indicate that this item should be at the bottom of the list.

This would be my solution:

void testIt() {
  final outOfBounds = order.length + 1;
  const description = 'description';
  items.sort(
    (lhs, rhs) {
      final lhsDesc = (lhs[description] as String).toLowerCase();
      final rhsDesc = (rhs[description] as String).toLowerCase();
      final lhsIndex =
          order.contains(lhsDesc) ? order.indexOf(lhsDesc) : outOfBounds;
      final rhsIndex =
          order.contains(rhsDesc) ? order.indexOf(rhsDesc) : outOfBounds;
      return lhsIndex.compareTo(rhsIndex);
    },
  );
}

And the result is:

[{id: 1, id2: 3, description: Top European}, {id: 0, id2: 5, description: Top USA}, {id: 6, id2: 6, description: Top Rest of the world}, {id: 2, id2: 2, description: Top A}, {id: 3, id2: 0, description: Top Z}, {id: 4, id2: 4, description: Top C}, {id: 5, id2: 1, description: Top D}]