Unmarshalling json to structure using json.RawMessage - json

Question

I need to unmarshal json object which may have the following formats:
Format1:
{
"contactType": 2,
"value": "0123456789"
}
Format2:
{
"contactType": "MobileNumber",
"value": "0123456789"
}
The structure I'm using for unmarshalling is:-
type Contact struct {
ContactType int `json:"contactType"`
Value string `json:"value"`
}
But this works only for format 1. I don't want to change the datatype of ContactType but I want to accommodate the 2nd format as well. I heard about json.RawMarshal and tried using it.
type Contact struct {
ContactType int
Value string `json:"value"`
Type json.RawMessage `json:"contactType"`
}
type StringContact struct {
Type string `json:"contactType"`
}
type IntContact struct {
Type int `json:"contactType"`
}
This gets the unmarshalling done, but I'm unable to set the ContactType variable which depends on the type of json.RawMessage. How do I model my structure so that this problem gets solved?

Answer 1

You will need to do the unmarshalling yourself. There is a very good article that shows how to use the json.RawMessage right and a number of other solutions to this very problem, Like using interfaces, RawMessage, implemention your own unmarshal and decode functions etc.
You will find the article here: JSON decoding in GO by Attila Oláh
Note: Attila has made a few errors on his code examples.
I taken the liberty to put together (using some of the code from Attila) a working example using RawMessage to delay the unmarshaling so we can do it on our own version of the Decode func.
Link to GOLANG Playground
package main
import (
"fmt"
"encoding/json"
"io"
)
type Record struct {
AuthorRaw json.RawMessage `json:"author"`
Title string `json:"title"`
URL string `json:"url"`
Author Author
}
type Author struct {
ID uint64 `json:"id"`
Email string `json:"email"`
}
func Decode(r io.Reader) (x *Record, err error) {
x = new(Record)
if err = json.NewDecoder(r).Decode(x); err != nil {
return
}
if err = json.Unmarshal(x.AuthorRaw, &x.Author); err == nil {
return
}
var s string
if err = json.Unmarshal(x.AuthorRaw, &s); err == nil {
x.Author.Email = s
return
}
var n uint64
if err = json.Unmarshal(x.AuthorRaw, &n); err == nil {
x.Author.ID = n
}
return
}
func main() {
byt_1 := []byte(`{"author": 2,"title": "some things","url": "https://stackoverflow.com"}`)
byt_2 := []byte(`{"author": "Mad Scientist","title": "some things","url": "https://stackoverflow.com"}`)
var dat Record
if err := json.Unmarshal(byt_1, &dat); err != nil {
panic(err)
}
fmt.Printf("%#s\r\n", dat)
if err := json.Unmarshal(byt_2, &dat); err != nil {
panic(err)
}
fmt.Printf("%#s\r\n", dat)
}
Hope this helps.