go - Assign additional field when unmarshalling JSON object to struct

What is the best way to assign additional field to struct and all it references sub-structure when encoding it from []byte and this field is not a part of unmarshalling []byte?

Let me clarify by example...

1. We have a few struct:

    type Update struct {
        UpdateID int32 `json:"update_id"`
        Message *Message `json:"message,omitempty"`
        ...
    }

    type Message struct {
        MessageID int32 `json:"message_id"`
        From *User `json:"from,omitempty"`
        Date int32 `json:"date"`
        Chat Chat `json:"chat"`
        ...
    }

2. Our core struct is APIClient and it has many methods for send something or get something (API wrapper)

    type API struct {
        Token string
        PollInt int32
        URL string

        client *http.Client
    }

    func(a *API) ... () {

    }

    func(a *API) ... () {

    }

    ...

3. Main loop poller make http request and return json response as "res" – []byte, so we can use json.Unmarshal to map it to special Update struct

    func (a *API) GetUpdates(ctx context.Context, gu *GetUpdates) ([]Update, error) {
        buf := bytes.Buffer{}
        err := json.NewEncoder(&buf).Encode(gu)
        if err != nil {
            return nil, fmt.Errorf("getupdates: %s", err)
        }

        res, err := a.DoRequest(&ctx, "getUpdates", &buf)
        if err != nil {
            return nil, fmt.Errorf("getupdates: %s", err)
        }

        var result []Update
        err = json.Unmarshal(res.Result, &result)
        if err != nil {
            return nil, fmt.Errorf("getupdates: %s", err)
        }

        return result, nil
    }

4. Is it any way to extend ALL structs in unmarhsaling chain for a additional field when use json.Unmarshal

    type Update struct {
        UpdateID int32 `json:"update_id"`
        Message *Message `json:"message,omitempty"`
        ...

        API `json:"-"`
    }

    type Message struct {
        MessageID int32 `json:"message_id"`
        From *User `json:"from,omitempty"`
        Date int32 `json:"date"`
        Chat Chat `json:"chat"`
        ...

        API `json:"-"`
    }

So we can do something like:

    var result []Update
    err = json.Unmarshal(res.Result, &result, api)
    if err != nil {
        return nil, fmt.Errorf("getupdates: %s", err)
    }

And then use it:

    result[0].RejectUpdate()
    result[0].Message.SendReply()
    ...
    func (u *Update) RejectUpdate(cause string) {
      m.API.SendMessage(u.Chat.ID, text)
      m.API.Reject(u.ID, cause)
    }
    func (m *Message) SendReply(text string) {
      m.API.SendMessage(m.Chat.ID, text)
    }

All struct will be extended by api (embedded struct) on unmarshaling...

My thoughts about solution:

1. Patch standard encoding/json library – not a good choice


2. Get custom encoding library and rewrite a little bit – questionable choice


3. Manually unmarshal all object – awful code

   type Bot struct {
        superCLient string
    }

    type Image struct {
        Name string
        Path string

        *Bot `json:"-"`
    }

    type FormFile struct {
        Img  *Image
        Name string

        *Bot `json:"-"`
    }

    func main() {
        data := []byte(`{"Img": {"Name": "Vi", "Path": "/etc/log"}, "Name": "Josh"}`)
        bot := &Bot{
            superCLient: "ClientExample",
        }
        var omg FormFile
        omg.CustomUnmarshal(data, bot)
        fmt.Println(omg)
    }

    func (ff *FormFile) CustomUnmarshal(data []byte, bot *Bot) error {
        var f map[string]*json.RawMessage
        json.Unmarshal(data, &f)

        var i Image
        i.CustomUnmarshal(*f["Img"], bot)
        ff.Img = &i

        json.Unmarshal(*f["Name"], ff.Name)

        ff.Bot = bot

        return nil
    }

    func (img *Image) CustomUnmarshal(data []byte, bot *Bot) error {
        err := json.Unmarshal(data, img)
        img.Bot = bot
        return err
    }