sort a mongodb query based on a field in a subdocument in Go

suppose I have the following struct

type Test struct {
  Title  string `bson:"title" json:"title"`
  Update Update `bson:"update" json:"update"`
}

type Update struct {
  Changes    []string  `bson:"change" json:"change"`
  UpdatedAt time.Time `bson:"updatedAt" json:"updatedAt"`
}

And suppose I want to sort my results in the Query by "update.updatedAt"

cs.Find(bson.M{title: "some title"}).Sort("-update.updatedAt").Limit(10).All(&results)

This query does not work as expected. I can't seem to find any documentation about how to sort a Query by a subdocument field. Intuitively, I was certain that my example would work. For reference, "-updatedAt" works just fine, but for reasons that aren't necessary to explain, I have to keep Update as a subdocument of Test.

Every example and Stack Overflow question I have seen so far involves the sorting and reordering of subdocuments, or arrays. In this case I don't care about the order of my subdocuments, and I'm not sorting by anything inside of an array. I merely want to sort my the documents by the date of a subdocument.

Is there a way to do this with the Go Mgo Query library?

I'm pretty sure it is something with your data structures because next code works as expected (you can test it on your PC)

package main

import (
	"fmt"
	"gopkg.in/mgo.v2"
	"gopkg.in/mgo.v2/bson"
	"log"
	"time"
)

type Test struct {
	Title  string `bson:"title" json:"title"`
	Update Update `bson:"update" json:"update"`
}

type Update struct {
	Changes   []string  `bson:"change" json:"change"`
	UpdatedAt time.Time `bson:"updatedAt" json:"updatedAt"`
}

func main() {
	session, err := mgo.Dial("mongodb://souser:123456@ds055855.mlab.com:55855/catalog")

	if err != nil {
		panic(err)
	}
	defer session.Close()
	session.SetMode(mgo.Monotonic, true)

	c := session.DB("catalog").C("sortable")

	/*
		// Fill collection with test data (test collection aready has 15 elements)
		for i := 0; i < 15; i++ {
			c.Insert(&Test{
				Title: "Title",
				Update: Update{
					Changes:   []string{fmt.Sprintf("Changes #%d", i)},
					UpdatedAt: time.Now(),
				},
			})
		}
	*/

	var results []Test

	err = c.Find(bson.M{"title": "Title"}).Sort("-update.updatedAt").Limit(10).All(&results)
	if err != nil {
		log.Fatal(err)
	}

	for _, e := range results {
		fmt.Println(e.Update.UpdatedAt)
	}
}

You need to pass subdocument fields comma(,) seperated like:

cs.Find(bson.M{title: "some title"}).Sort("update","-updatedAt").Limit(10).All(&results)