mgo.go

package main

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

func withUpsert(c *mgo.Collection) {
	_, err := c.Upsert(bson.M{"_id": 1, "x": 2}, bson.M{"$set": bson.M{"x": 3}})
	fmt.Printf("ISDUP: %v - %s\n", mgo.IsDup(err), err)
	// OUTPUT - ISDUP: true - E11000 duplicate key error index: test-mgo-dup.a.$_id_ dup key: { : 1 }
}

func withApply(c *mgo.Collection) {
	_, err := c.Find(bson.M{"_id": 1, "x": 2}).Apply(mgo.Change{
		Update: bson.M{"$set": bson.M{"x": 3}},
		Upsert: true,
	}, nil)
	fmt.Printf("ISDUP: %v - %s\n", mgo.IsDup(err), err)
	// OUTPUT - ISDUP: false - not found
}

func main() {
	conn, err := mgo.Dial("127.0.0.1:27017")
	if err != nil {
		panic(err)
	}
	c := conn.DB("test-mgo-dup").C("a")
	c.DropCollection()
	err = c.Insert(bson.M{"_id": 1, "x": 1})
	if err != nil {
		panic(err)
	}
	withUpsert(c)
	withApply(c)
}