mgo findAndModify change _id error

main.go

package main

import (
	"log"

	"gopkg.in/mgo.v2"
	"gopkg.in/mgo.v2/bson"
)

func main() {
	s, err := mgo.Dial("127.0.0.1:27017")
	if err != nil {
		log.Fatal(err)
	}
	coll := s.DB("test").C("test")
	// do insert
	err = coll.Insert(bson.M{"_id": "to_change_id"})
	if err != nil {
		log.Fatal(err)
	}
	// do change _id
	var ret interface{}
	_, err = coll.FindId("to_change_id").Apply(mgo.Change{
		Update: bson.M{"$set": bson.M{"_id": "change_to_new_id"}},
	}, &ret)
	if err != nil {
		log.Fatal(err)
	}
	log.Println(ret)
}

2016/01/14 10:23:22 After applying the update to the document {_id: "to_change_id" , ...}, the (immutable) field '_id' was found to have been altered to _id: "change_to_new_id"