How can I remove duplicate list then get the latest ID?

Question

I have this List<Map> :

[
  {msg_id: 1, from_id: 10, to_id: 20, text: 'Some text123'},
  {msg_id: 2, from_id: 20, to_id: 10, text: 'Some text321'},
  {msg_id: 3, from_id: 10, to_id: 20, text: 'Some text again'},
  {msg_id: 4, from_id: 5, to_id: 15, text: 'Hello World'},
  {msg_id: 5, from_id: 15, to_id: 5, text: 'Hello World'}
];

Then I need to get last message by msg_id with different chat rooms like this :

[
  {msg_id: 3, from_id: 10, to_id: 20, text: 'Some text again'},
  {msg_id: 5, from_id: 15, to_id: 5, text: 'Hello World'}
];

I've tried distinct() and set() but I still confuse. What should I do?

Solution

Since what you want is "group by chat_room_id", we need to generate it first from from_id and to_id. I assume your list are already sorted by msg_id. I generate the chat_room_id by joining from_id and to_id after it sorted. The reason why we need to sort them first because from_id 10, to_id 20 is the same chat room as from_id 20, to_id 10.

  List<Map> data = [
    {"msg_id": 1, "from_id": 10, "to_id": 20, "text": 'Some text123'},
    {"msg_id": 2, "from_id": 20, "to_id": 10, "text": 'Some text321'},
    {"msg_id": 3, "from_id": 10, "to_id": 20, "text": 'Some text again'},
    {"msg_id": 4, "from_id": 5, "to_id": 15, "text": 'Hello World'},
    {"msg_id": 5, "from_id": 15, "to_id": 5, "text": 'Hello World'}
  ];
​
  Map<String, Map> perRoom = {};
​
  data.forEach((d) {
    // getting room id
    List<int> roomIdList = [d["from_id"], d["to_id"]];
    // need to be sorted since from_id 10, to_id 20 is the same room as from_id 20, to_id 10
    roomIdList.sort();
    String roomId = roomIdList.join('-');
    perRoom[roomId] = d;
  });
​
  // convert Map<String, Map> back into List<Map>
  List<Map> onlyLatest = perRoom.values.toList();
​
  print(onlyLatest);

on Dartpad

If your list comes from query, I really recommend you to have chat_room_id in the database since you can just use something like GROUP BY and avoid getting a lot of data from the DB.

Comments

[DK_bhai]

What are you considering duplicate here?

[husainazkas]

@D'Kayd chat room by from_id and to_id

[husainazkas]

so we got the room id with String type?

[Sudhanta Suryaputra]

@husainazkas yess. I can't think another type that more suitable for this. But of course it's up to you how you want to generate the room_id. Just make sure to make both directions (from 10 to 20 and from 20 to 10) generate same room_id