golang之mgo自增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}