Add custom field to JSON in Go

add_custom_field_to_json.go

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
)

type D struct {
	ID  string
	Rev string
	Doc interface{}
}

func (d D) MarshalJSON() ([]byte, error) {
	out := make(map[string]interface{})
	dValue := reflect.Indirect(reflect.ValueOf(d.Doc))
	dType := dValue.Type()
	for i := 0; i < dType.NumField(); i++ {
		field := dType.Field(i)
		name := dType.Field(i).Name
		jsonKey := field.Tag.Get("json")
		if jsonKey == "" {
			jsonKey = name
		}
		out[jsonKey] = dValue.FieldByName(name).Interface()
	}
	out["_id"] = d.ID
	out["_rev"] = d.Rev
	
	return json.Marshal(out)
}

type Document struct {
	Name string `json:"name"`
}

func main() {
	b := Document{}
	b.Name = "Bob"
	
	d := D{}
	d.ID = "aiueo"
	d.Rev = "1-1e895f6f-aa12-4b72-b3d6-9924b6d0678a"
	d.Doc = b

	jb, err := json.MarshalIndent(&d, "", "  ")
	if err != nil {
		fmt.Println(err)
	} else {
		fmt.Println(string(jb))
	}

}