golang of mgo increment id

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

// id从1开始递增
func GenId(db *mgo.Database) (int64, error) {
   IDInt64 := struct {
      Value int64 `bson:"max_id"`
   }{Value: 1}
   _, err := db.C("gen_id").Find(bson.M{}).Apply(mgo.Change{Update: bson.M{"$inc": IDInt64}, Upsert: true, ReturnNew: true}, &IDInt64)
   return IDInt64.Value, err
}