Go Struct嵌入与mgo持久化：实现BSON文档的扁平化存储

package main

import (
    "fmt"
    "encoding/json"
)

type Square struct {
    Length int
    Width  int
}

type Cube struct {
    Square // 嵌入Square结构体
    Depth  int
}

func main() {
    c := new(Cube)
    c.Length = 2 // 直接访问嵌入结构体的字段
    c.Width = 3
    c.Depth = 4

    // 使用json.Marshal时的行为
    b, err := json.Marshal(c)
    if err != nil {
        panic(err)
    }
    fmt.Println("JSON Marshal Output:", string(b))
    // 预期输出: {"Length":2,"Width":3,"Depth":4}
}

{
    "Square": {
        "Length": 2,
        "Width": 3
    },
    "Depth": 4
}

inline     Inline the field, which must be a struct or a map,
           causing all of its fields or keys to be processed as if
           they were part of the outer struct. For maps, keys must
           not conflict with the bson keys of other struct fields.

type Cube struct {
    Square `bson:",inline"` // 添加inline标签
    Depth  int
}

package main

import (
    "fmt"
    "log"
    "time"

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

// Square 定义一个基础的二维形状
type Square struct {
    Length int `bson:"length"` // 定义BSON字段名
    Width  int `bson:"width"`
}

// Cube 嵌入Square并添加深度
type Cube struct {
    ID     bson.ObjectId `bson:"_id,omitempty"` // MongoDB文档ID
    Square `bson:",inline"`                    // 使用inline标签扁平化Square字段
    Depth  int           `bson:"depth"`
}

func main() {
    // 1. 连接MongoDB
    session, err := mgo.Dial("mongodb://localhost:27017") // 请确保MongoDB服务已运行
    if err != nil {
        log.Fatalf("Failed to connect to MongoDB: %v", err)
    }
    defer session.Close()

    // 设置会话模式，确保读写一致性
    session.SetMode(mgo.Monotonic, true)

    // 获取数据库和集合
    c := session.DB("testdb").C("cubes")

    // 2. 创建一个Cube实例
    cube := Cube{
        ID:    bson.NewObjectId(),
        Depth: 4,
    }
    cube.Length = 2 // 直接设置嵌入结构体的字段
    cube.Width = 3

    // 3. 插入或更新文档 (Upsert)
    // 使用bson.M{"_id": cube.ID}作为查询条件，&cube作为更新内容
    _, err = c.Upsert(bson.M{"_id": cube.ID}, &cube)
    if err != nil {
        log.Fatalf("Failed to upsert document: %v", err)
    }
    fmt.Printf("Document with ID %s upserted successfully.\n", cube.ID.Hex())

    // 4. 从MongoDB中查询并验证
    var retrievedCube Cube
    err = c.FindId(cube.ID).One(&retrievedCube)
    if err != nil {
        log.Fatalf("Failed to retrieve document: %v", err)
    }

    fmt.Printf("\nRetrieved Cube:\n")
    fmt.Printf("  ID: %s\n", retrievedCube.ID.Hex())
    fmt.Printf("  Length: %d\n", retrievedCube.Length)
    fmt.Printf("  Width: %d\n", retrievedCube.Width)
    fmt.Printf("  Depth: %d\n", retrievedCube.Depth)

    // 可选：打印BSON文档的原始形式 (需要通过MongoDB客户端查看)
    fmt.Println("\nTo verify the flattened structure, please check MongoDB using a client (e.g., MongoDB Compass or mongo shell).")
    fmt.Println("Expected MongoDB document structure:")
    fmt.Println(`{
    "_id": ObjectId("..."),
    "length": 2,
    "width": 3,
    "depth": 4
}`)

    // 清理数据 (可选)
    // err = c.RemoveId(cube.ID)
    // if err != nil {
    //  log.Printf("Failed to remove document: %v", err)
    // } else {
    //  fmt.Printf("\nDocument with ID %s removed.\n", cube.ID.Hex())
    // }
}

{
    "_id": ObjectId("65c6f3d1e1b2c3d4e5f6a7b8"), // 示例ID
    "length": 2,
    "width": 3,
    "depth": 4
}