json unmarshal embedded struct

Question

I would like to unmarshal to struct Outer defined as:

type Outer struct {
    Inner
    Num int
}

type Inner struct {
    Data string
}
func (i *Inner) UnmarshalJSON(data []byte) error {
    i.Data = string(data)
    return nil
}

Using json.Unmarshal(data, &Outer{}) seems only to use Inner's UnmarshalJSON and ignores the Num field: https://play.golang.org/p/WUBfzpheMl

I have an unwieldy solution where I set the Num field manually, but I was wondering if anybody had a cleaner or simpler way to do it.

Thanks!

Answer 1

This is happening because Inner is being embedded in Outer. That means when json library calls unmarshaler on Outer, it instead ends up calling it on Inner.

Therefore, inside func (i *Inner) UnmarshalJSON(data []byte), the data argument contains the entire json string, which you are then processing for Inner only.

You can fix this by making Inner explicit field in Outer

Outer struct {
    I Inner // make Inner an explicit field
    Num int `json:"Num"`
}

Working example

Answer 2

You shouldn't need an unmarshalling function

https://play.golang.org/p/-HZwX5-rPD

Edit: Here is a more complete example

https://play.golang.org/p/oz5kiwy3_K

Answer 3

Just delete UnmarshalJSON in your example because it's used in unmarshaling of Outer since Inner is inlined. Otherwise, you need to override it if you want to do something custom.

https://play.golang.org/p/D6V6vKpx9J

Answer 4

Actually you not need explicit field, you need properly Marshal/UnMarshal

Example: https://play.golang.org/p/mWPM7m44wfK

package main

import (
    "encoding/json"
    "fmt"
)

type Outer struct {
    Inner
    Num int `json:"Num"`
}

type Inner struct{ Data string }

type InnerRaw struct {Data string}

func (i *Inner) UnmarshalJSON(data []byte) error {
    ir:=&InnerRaw{}
    json.Unmarshal(data, ir)
    i.Data = ir.Data
    return nil
}

func main() {
    x := Outer{}
    data := []byte(`{"Num": 4, "Data":"234"}`)
    _ = json.Unmarshal(data, &x)
    fmt.Printf("%+v\n", x)
    js, _:=json.Marshal(x)
    fmt.Printf("JSON:%s", string(js))
}