Idiomatic way to extend a struct when marshalling to Json

extend_struct_json.go

package main

// This example shows how to extend a struct for marshalling, e.g. to Json to allow private fields 
// to be exposed or for adding additonal fields that are populated at runtime.

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

type Job struct {
	Job      string
	Started  time.Time
	Finished time.Time
	private  string
}

func (j *Job) Duration() time.Duration {
	if j.Finished.IsZero() {
		return time.Since(j.Started)
	} else {
		return j.Finished.Sub(j.Started)
	}
}

func (j *Job) MarshalJSON() ([]byte, error) {
	// introduce a new temproary struct
	type temp struct {
		Job             // Job as an anonymous field
		Private  string // this will be set to the job's private field
		Duration string // this is a new field that will be the result of calling a function on the orginal job instance
	}

	t := temp{
		*j,                    // our base job
		j.private,             // Sets Private to j.private
		j.Duration().String(), //Sets the Duration to the result of a job.method
	}

	return json.Marshal(t)
}

func main() {
	// create job
	j := Job{
		Job:      "My Job",
		Started:  time.Now(),
		Finished: time.Now().Add(10 * time.Second),
		private:  "private string",
	}

	// marshal to json
	b, _ := json.Marshal(&j)

	// print out json
	fmt.Println(string(b))
}