深入解析：mgo中Go结构体_id字段检索失败的常见原因与解决方案

import (
    "fmt"
    "log"

    "gopkg.in/mgo.v2"
    "gopkg.in/mgo.v2/bson"
)

type Room struct {
    Id   bson.ObjectId `json:"Id"bson:"_id"` // 注意这里json和bson标签之间没有空格
    Name string        `json:"Name" bson:"name"`
}

// 假设 RoomCollection 已经是一个 mgo.Collection 实例
var RoomCollection *mgo.Collection // 实际应用中会通过会话获取

func init() {
    // 示例初始化，实际应用中应有适当的错误处理
    session, err := mgo.Dial("mongodb://localhost:27017/testdb")
    if err != nil {
        log.Fatalf("Failed to connect to MongoDB: %v", err)
    }
    session.SetMode(mgo.Monotonic, true)
    RoomCollection = session.DB("testdb").C("rooms")
}

func main() {
    // 插入文档
    room := &Room{Id: bson.NewObjectId(), Name: "test"}
    if err := RoomCollection.Insert(room); err != nil {
        log.Fatalf("Failed to insert room: %v", err)
    }
    fmt.Printf("Inserted Room: %+v\n", room)

    // 尝试通过 _id 查询
    roomZ := &Room{}
    if err := RoomCollection.Find(bson.M{"_id": room.Id}).One(roomZ); err != nil {
        log.Fatalf("Failed to retrieve room by _id: %v", err) // 这里会抛出 "not found" 错误
    }
    fmt.Printf("Retrieved Room by _id: %+v\n", roomZ)
}

type Room struct {
    Id   bson.ObjectId `json:"Id" bson:"_id"` // 正确的写法：json和bson标签之间有空格
    Name string        `json:"Name" bson:"name"`
}

package main

import (
    "fmt"
    "log"

    "gopkg.in/mgo.v2"
    "gopkg.in/mgo.v2/bson"
)

// Room 结构体，修正了标签格式
type Room struct {
    Id   bson.ObjectId `json:"Id" bson:"_id"` // 正确的写法：json和bson标签之间有空格
    Name string        `json:"Name" bson:"name"`
}

var RoomCollection *mgo.Collection

func init() {
    session, err := mgo.Dial("mongodb://localhost:27017/testdb")
    if err != nil {
        log.Fatalf("Failed to connect to MongoDB: %v", err)
    }
    session.SetMode(mgo.Monotonic, true)
    RoomCollection = session.DB("testdb").C("rooms")

    // 清理旧数据，确保示例环境干净
    if _, err := RoomCollection.RemoveAll(bson.M{}); err != nil {
        log.Printf("Failed to clean up collection: %v", err)
    }
}

func main() {
    // 插入文档
    room := &Room{Id: bson.NewObjectId(), Name: "test"}
    if err := RoomCollection.Insert(room); err != nil {
        log.Fatalf("Failed to insert room: %v", err)
    }
    fmt.Printf("Inserted Room: %+v\n", room)

    // 尝试通过 _id 查询 (现在应该成功)
    roomZ := &Room{}
    if err := RoomCollection.Find(bson.M{"_id": room.Id}).One(roomZ); err != nil {
        log.Fatalf("Failed to retrieve room by _id: %v", err) // 不再抛出 "not found" 错误
    }
    fmt.Printf("Retrieved Room by _id: %+v\n", roomZ)

    // 再次验证，使用任意查询 (仍然成功)
    roomX := &Room{}
    if err := RoomCollection.Find(bson.M{}).One(roomX); err != nil {
        log.Fatalf("Failed to retrieve any room: %v", err)
    }
    fmt.Printf("Retrieved any Room: %+v\n", roomX)
}