sort in populate does not work well

[hongbo-miao]

Originally asked on Stack Overflow.

My MongoDB version 3.2, mongoose version is 4.6.0

These are my schemas:

    // chat
    const chatSchema = new mongoose.Schema({
      users: [{ type: mongoose.Schema.Types.ObjectId, ref: 'User', required: true }],
      lastMessage:  { type: mongoose.Schema.Types.ObjectId, ref: 'Message' }
    });
    export const ChatModel = mongoose.model('Chat', chatSchema);

    // message
    const messageSchema = new mongoose.Schema({
      user: { type: mongoose.Schema.Types.ObjectId, ref: 'User', required: true },
      chat: { type: mongoose.Schema.Types.ObjectId, ref: 'Chat', required: true },
      text: { type: String, required: true },
      timestamp: { type: Date, default: Date.now }
    });
    export const MessageModel = mongoose.model('Message', messageSchema);

I want to sort based on lastMessage's timestamp in a desc order. I tried these three

    ChatModel
      .find({}, 'lastMessage')
      .populate('lastMessage', 'timestamp', null, { sort: { timestamp: -1 }})
      .exec()
      .then(chats => console.log(chats))

    ChatModel
      .find({}, 'lastMessage')
      .populate({
        path: 'lastMessage',
        select: 'timestamp',
        options: { sort: { timestamp: -1 }}
      })
      .exec()
      .then(chats => console.log(chats))

    ChatModel
      .find({}, 'lastMessage')
      .populate('lastMessage', 'timestamp')
      .sort({ 'lastMessage.timestamp': -1 })
      .exec()
      .then(chats => console.log(chats))

Also, no matter I use -1 or 1, 'desc', or 'asc' for timestamp, it always gives me same results:

  [{
    _id: 57c8a682cde8baf5c36eb1fc,
    lastMessage: {
      _id: 57c8baa29a293eace7f9be15,
      timestamp: 2016-09-01T23:32:50.344Z
    }
  }, {
    _id: 57c8a6d0cde8baf5c36eb1fe,
    lastMessage: {
      _id: 57c8fabb4362b3c25d828774,
      timestamp: 2016-09-02T04:06:19.421Z
    }
  }]

[vkarpov15]

Not supported, you should store a lastMessageTimestamp in chat schema if you want to sort chats by the last message timestamp. That approach is much more performant because you can use indexes.

[iamprahladsinghnegi]

Hi @vkarpov15
Is this feature supported in mongoosev5.9.7?

[vkarpov15]

@iamprahladsinghnegi no