Pretty cool trick to map Go struct to and from arbitrary JSON

custom.go

/* Original code posted here https://stackoverflow.com/questions/27492888/how-to-map-json-objects-with-dynamic-fields-to-go-structs */

package main

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

type Random struct {
	CustomFields map[string]interface{}
}

func (r *Random) MarshalJSON() ([]byte, error) {
    
    	randmap := make(map[string]interface{})

    	// mapping the first-level JSON key
    	for k, v := range r.CustomFields {
        	randmap[k] = v
    	}

    	return json.Marshal(randmap)
}

func (r *Random) UnmarshalJSON(data []byte) error {
	var randmap map[string]interface{}
	
	if r == nil {
		return errors.New("RawString: UnmarshalJSON on nil")
	}
	
	if err := json.Unmarshal(data, &randmap); err != nil {
		return err
	}
	
	for k, v := range randmap {
		r.CustomFields[k] = v
	}
	return nil
}

var uj = []byte(`{"name": "foo", "age": 300, "gigs": ["one", "two", 3.21]}`)

func main() {
 	r := Random{
		CustomFields: map[string]interface{}{
			"gaga": "ball",
			"bambi": 2,
			"darn": []interface{}{"one", 2, 3.0},
		},
	}
	
	// Get a JSON from the Random struct
	result, _ := r.MarshalJSON()
	
	// This will unmarshal the uj blob to a map in CustomField
	r.UnmarshalJSON(uj)
	
	fmt.Println(string(result))
	
	// marshal back to the struct
	//r.UnmarshalJSON(result)
	
	fmt.Println(r.CustomFields)
}