sort in populate does not work (Mongoose)

问题

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
    }
  }]

What may cause this? Thanks

---

UPDATE1:

It seems like a bug of Mongoose.

Please track this issue on GitHub.

---

UPDATE2: It said not supported. But I don't know why... Is using sort in populate wrong for this case?

回答1:

The documentation (http://mongoosejs.com/docs/api.html#document_Document-populate) states something like:

Chat.find().populate({
    path: 'lastMessage'
  , select: 'timestamp'
  , options: { sort: { timestamp: -1 }}
}).exec(function (err, chats) {
  //do something
})

If this is not working first check it with for example text, to see if it's a datetime sort issue

UPDATE: When i re-read your question i noticed the issue, you want to sort all messages based on the last message timestamp. There is no need to sort in the population because it only returns 1 item (there is no array of last messages).

So the syntax would be:

Chat.find().populate({
        path: 'lastMessage'
      , select: 'timestamp'})
.sort('lastMessage.timestamp')
.exec(function (err, chats) {
      //do something
    })

回答2:

Try this..

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

来源：https://stackoverflow.com/questions/39285102/sort-in-populate-does-not-work-mongoose