重命名JSON字段

Question

We want to rename a JSON field value to v in production. Till all our users use the new struct, we would continue to get old JSON structs into our code. So we want to handle this as well.

If you notice, First is the original structure, Second is the new structure. To handle both these structures, I have created a MyStruct and based on version, I copy the OldValue into Value

if m.Version <= 1 {
    m.Value = m.OldValue
}

Is there a better way to handle this, instead of my code.

Go Playground Link

package main

import "fmt"
import "encoding/json"
import "log"

type First struct {
    Version int `json:"version"`
    Value   int `json:"value"`
}

type Second struct {
    Version int `json:"version"`
    Value   int `json:"v"`
}

type MyStruct struct {
    Version  int `json:"version"`
    OldValue int `json:"value"`
    Value    int `json:"v"`
}

func main() {
    oldValue := []byte(`{"version":1, "value":5}`)
    newValue := []byte(`{"version":2, "v":7}`)

    var m MyStruct

    err := json.Unmarshal(newValue, &m)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("New Struct")
    fmt.Println(m.Value)

    err = json.Unmarshal(oldValue, &m)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("Old Struct")
    if m.Version <= 1 {
        m.Value = m.OldValue
    }
    fmt.Println(m.Value)

}

Answer 1

EDIT: You can actually do it with one unmarshaling, albeit you'll need another type:

type Second struct {
    Version int `json:"version"`
    Value   int `json:"v"`
}

type SecondWithOldValue struct {
    OldValue int `json:"value"`
    Second
}

type MyStruct SecondWithOldValue

func (v *MyStruct) UnmarshalJSON(b []byte) error {
    if err := json.Unmarshal(b, (*SecondWithOldValue)(v)); err != nil {
        return err
    }

    if v.Version <= 1 {
        v.Value = v.OldValue
    }
    return nil
}

Playground: https://play.golang.org/p/yII-ncxnU4.

Old answer below.

---

If you're OK with double unmarshaling, you can do it like this:

type Second struct {
    Version int `json:"version"`
    Value   int `json:"v"`
}

type MyStruct struct {
    Second
}

func (v *MyStruct) UnmarshalJSON(b []byte) error {
    if err := json.Unmarshal(b, &v.Second); err != nil {
        return err
    }

    if v.Version <= 1 {
        var oldV struct{ Value int }
        if err := json.Unmarshal(b, &oldV); err != nil {
            return err
        }
        v.Value = oldV.Value
    }
    return nil
}

First, unmarshal into the inner struct, check version, and if it's an old one, get the old value.

Playground: https://play.golang.org/p/AaULW6vJz_.