Marshal interface{} into json - json

Question

I want to generate a json that's something like this:
{
"A": 1,
"B": "bVal",
"C": "cVal"
}
But I want to keep my code generic enough that I can replace the key-pair "C" with any other type of json object that I want. I tried doing something like what I have below:
type JsonMessage struct {
A int `json:"A"`
B string `json:"B,omitempty"`
genericObj interface{}
}
type JsonObj struct {
C string `json:"C"`
}
func main() {
myJsonObj := JsonObj{C: "cVal"}
myJson := JsonMessage{A: 1, B: "bValue", genericObj: myJsonObj}
body, _ := json.Marshal(myJson)
fmt.Print(body)
}
But the output is just this:
{
"A": 1,
"B": "bVal"
}
Is there a different way to approach this?

Answer 1

This is precisely why json.RawMessage exists.
Here is an example straight from the Go docs:
package main
import (
"encoding/json"
"fmt"
"os"
)
func main() {
h := json.RawMessage(`{"precomputed": true}`)
c := struct {
Header *json.RawMessage `json:"header"`
Body string `json:"body"`
}{Header: &h, Body: "Hello Gophers!"}
b, err := json.MarshalIndent(&c, "", "\t")
if err != nil {
fmt.Println("error:", err)
}
os.Stdout.Write(b)
}
Here is the output:
{
"header": {
"precomputed": true
},
"body": "Hello Gophers!"
}
Go Playground: https://play.golang.org/p/skYJT1WyM1C
Of course you can marshal a value before time to get the raw bytes in your case.

Answer 2

package main
import (
"encoding/json"
"fmt"
)
type JsonMessage struct {
A int `json:"A"`
B string `json:"B,omitempty"`
// simply make this field capitalized,and it will be included in the
// json string
GenericObj interface{}
}
type JsonObj struct {
C string `json:"C"`
}
func main() {
myJsonObj := JsonObj{C: "cVal"}
myJson := JsonMessage{A: 1, B: "bValue", GenericObj: myJsonObj}
body, _ := json.Marshal(myJson)
fmt.Println(string(body))
}
{"A":1,"B":"bValue","GenericObj":{"C":"cVal"}}