实际情况
默认情况下，mongo使用_id自动生成uniq id，而且在mongo自带的命令里，无法指定一个自增字段。自增字段在多线程时必须是原子性的，这在大数据情况下很难实现伸缩性(scalability)。
Generally in MongoDB, you would not use an auto-increment pattern for
the _id field, or any field, because it does not scale for databases
with large numbers of documents. Typically the default value ObjectId
is more ideal for the _id.
不过好消息是，_id不一定非得是ObjectId类型，它可以是任何类型。而且，在mongo里面，有一个findAndModify命令是原子性的。我们可以使用它实现auto increment ID。
在golang的mgo库里，Apply命令可以实现findAndModify功能
具体策略和代码
策略
假设我们要把test表的_id字段设置为自增的，我们需要先创建一个counter表，这个表里面保存一个seq字段，通过在seq上使用Apply，获取新doc的seq值作为test表中下一个id的值。
代码如下：
package main
import (
"fmt"
"gopkg.in/mgo.v2"
"gopkg.in/mgo.v2/bson"
"log"
"math/rand"
"sync"
)
func init() {
log.SetFlags(log.Lshortfile | log.LstdFlags)
}
func main() {
session, err := mgo.Dial("usr:pwd@127.0.0.1:27017/dbname")
if err != nil {
log.Fatal("无法打开MongoDB！")
return
}
defer session.Close()
clt := session.DB("dbname").C("test")
getNextSeq := func() func() int {
counter := session.DB("dbname").C("counter")
cid := "counterid"
return func() int {
change := mgo.Change{
Update: bson.M{"$inc": bson.M{"seq": 1}},
Upsert: true,
ReturnNew: true,
}
doc := struct{ Seq int }{}
if _, err := counter.Find(bson.M{"_id": cid}).Apply(change, &doc); err != nil {
panic(fmt.Errorf("get counter failed:", err.Error()))
}
log.Println("seq:", doc)
return doc.Seq
}
}()
wg := sync.WaitGroup{}
// 创建10个 go routine模拟多线程环境
for i := 0; i < 10; i++ {
wg.Add(1)
go func(i int) {
_row := map[string]interface{}{
"_id": getNextSeq(),
"username": fmt.Sprintf("name%2d", i),
"telephone": fmt.Sprintf("tel%4d", rand.Int63n(1000))}
if err := clt.Insert(_row); err != nil {
log.Printf("insert %v failed: %v\n", _row, err)
} else {
log.Printf("insert: %v success!\n", _row)
}
wg.Done()
}(i)
}
wg.Wait()
}
输出：
2016/04/28 17:05:28 main.go:37: seq: {201}
2016/04/28 17:05:28 main.go:54: insert: map[_id:201 username:name 9 telephone:tel 410] success!
2016/04/28 17:05:28 main.go:37: seq: {204}
2016/04/28 17:05:28 main.go:37: seq: {203}
2016/04/28 17:05:28 main.go:37: seq: {202}
2016/04/28 17:05:28 main.go:37: seq: {206}
2016/04/28 17:05:28 main.go:37: seq: {205}
2016/04/28 17:05:28 main.go:54: insert: map[_id:202 username:name 5 telephone:tel 51] success!
2016/04/28 17:05:28 main.go:54: insert: map[_id:204 username:name 2 telephone:tel 551] success!
2016/04/28 17:05:28 main.go:54: insert: map[telephone:tel 821 _id:203 username:name 1] success!
2016/04/28 17:05:28 main.go:37: seq: {207}
2016/04/28 17:05:28 main.go:54: insert: map[_id:205 username:name 6 telephone:tel 320] success!
2016/04/28 17:05:28 main.go:54: insert: map[_id:206 username:name 4 telephone:tel 937] success!
2016/04/28 17:05:28 main.go:54: insert: map[_id:207 username:name 3 telephone:tel 758] success!
2016/04/28 17:05:28 main.go:37: seq: {208}
2016/04/28 17:05:28 main.go:54: insert: map[username:name 7 telephone:tel 148 _id:208] success!
2016/04/28 17:05:28 main.go:37: seq: {209}
2016/04/28 17:05:28 main.go:54: insert: map[_id:209 username:name 0 telephone:tel 216] success!
2016/04/28 17:05:28 main.go:37: seq: {210}
2016/04/28 17:05:28 main.go:54: insert: map[_id:210 username:name 8 telephone:tel 449] success!
mongo查询结果
db.test.find({_id:{$gt:200}})
{ "_id" : 201, "username" : "name 9", "telephone" : "tel 410" }
{ "_id" : 202, "username" : "name 5", "telephone" : "tel 51" }
{ "_id" : 203, "username" : "name 1", "telephone" : "tel 821" }
{ "_id" : 204, "username" : "name 2", "telephone" : "tel 551" }
{ "_id" : 205, "username" : "name 6", "telephone" : "tel 320" }
{ "_id" : 206, "username" : "name 4", "telephone" : "tel 937" }
{ "_id" : 207, "username" : "name 3", "telephone" : "tel 758" }
{ "_id" : 208, "username" : "name 7", "telephone" : "tel 148" }
{ "_id" : 209, "username" : "name 0", "telephone" : "tel 216" }
{ "_id" : 210, "username" : "name 8", "telephone" : "tel 449" }