Mongoose, get latest message from every conversation

Question

how to get the latest message from every conversation

const chatSchema = new schema({
  from: { type: schema.Types.ObjectId, ref: "userSchema" },
  to: { type: schema.Types.ObjectId, ref: "userSchema" },
  text: String,
  date: { type: Date, default: Date.now },
});

Answer 1

Here, I suppose you want to get the latest messages for a certain user (say User John). Firstly, I will recommend you add a further field in your chat Object, say chatId that is always the same between a pair of users say John -> Anna and vice-versa. This way it will be a lot easier to track messages.

John's Object Id: johnId, chat Id: chatId

db.collection.aggregate([
             {$match: {$or: [{to: johnId}, {from: johnid}]},   // get messages to and fro user
             {$sort: {date: -1}},  // sort the messages in desc order
             {$group: {_id: "$chatId", doc: "$first"}} // group messages by chatId and get last message
             {$replaceRoot: {newRoot: "$doc"}} // replace the last message with root
            ]);