Question

使用mgo将Go struct类型的对象作为文档插入MongoDB数据库中的集合时，字段的名称是否从大写字母更改自动小写？
如果是，为什么mgo的插入方法会这样做？

感谢。

这是我的Go程序，它使用mgo在MongoDB服务器中执行插入和查询操作

package main

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

type Record struct {
    Dimension_id  int
    Attribute string
    Hour string
    Frequency_count int
}

func main(){

    session, err := mgo.Dial("localhost")
    if err != nil {
        panic(err)
    }
    defer session.Close()

    c := session.DB("TVFS").C("c20160712")

    // insert
    doc := Record{2, "good", "20160712_06", 100}
    err = c.Insert(&doc)
    if err != nil {
        panic(err)
    }

    // query
    result := Record{}
    err = c.Find(bson.M{"Dimension_id": 2, "Attribute": "good", "Hour": "20160712_06" }).One(&result) // no matching document
    // err = c.Find(bson.M{"dimension_id": 2, "attribute": "good", "hour": "20160712_06" }).One(&result) // one matching document
    if err != nil {
        panic(err)
    }
    fmt.Printf("count:%d\n", result.Frequency_count)

}

运行其编译程序的输出指出

$ ./minimal-example 
panic: not found

goroutine 1 [running]:
panic(0x61d500, 0xc82000a880)
    /usr/local/go/src/runtime/panic.go:481 +0x3e6
main.main()
    /home/t/program_files/mywork/go/src/tvfs/mongodb/minimal-example.go:38 +0x701

通过从shell连接到MongoDB服务器，我发现插入文档中的字段名称已从首字母大写变为小写首字母

$ mongo
MongoDB shell version: 3.2.8
connecting to: test
Server has startup warnings: 
2016-08-04T11:58:21.138-0400 I CONTROL  [initandlisten] 
2016-08-04T11:58:21.138-0400 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/enabled is 'always'.
2016-08-04T11:58:21.138-0400 I CONTROL  [initandlisten] **        We suggest setting it to 'never'
2016-08-04T11:58:21.138-0400 I CONTROL  [initandlisten] 
2016-08-04T11:58:21.139-0400 I CONTROL  [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2016-08-04T11:58:21.139-0400 I CONTROL  [initandlisten] **        We suggest setting it to 'never'
2016-08-04T11:58:21.139-0400 I CONTROL  [initandlisten] 
> show dbs
TVFS   0.000GB
local  0.000GB
> use TVFS
switched to db TVFS
> show collections
c20160712
> db.c20160712.find()
{ "_id" : ObjectId("57a3978491c3b3a393e9be2d"), "dimension_id" : 2, "attribute" : "good", "hour" : "20160712_06", "frequency_count" : 100 }

所以在我的Go计划中，我改变了

    err = c.Find(bson.M{"Dimension_id": 2, "Attribute": "good", "Hour": "20160712_06" }).One(&result) // no matching document

是

    err = c.Find(bson.M{"dimension_id": 2, "attribute": "good", "hour": "20160712_06" }).One(&result) // one matching document

并且有一个匹配的文档

$ ./minimal-example 
count:100

Answer 1

字段名称根据mgo/bson documentation小写：

小写字段名称用作每个导出字段的键，但可以使用相应的字段标记更改此行为。

使用bson字段标记覆盖mgo / bson对结构名称的处理。以下是匹配struct字段名称所需的标记：

type Record struct {
   Dimension_id  int     `bson:"Dimension_id"`
   Attribute string      `bson:"Attribute"`
   Hour string           `bson:"Hour"`
   Frequency_count int   `bson:"Frequency_count"`
}

mgo的insert方法是否将字段名称从大写更改为小写？

1 个答案: