Custom JSON Marshalling in Go

Go’s encoding/json package makes it really easy to marshal structs to JSON data.

package main

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

type MyUser struct {
	ID       int64     `json:"id"`
	Name     string    `json:"name"`
	LastSeen time.Time `json:"lastSeen"`
}

func main() {
	_ = json.NewEncoder(os.Stdout).Encode(
		&MyUser{1, "Ken", time.Now()},
	)
}

Output:

{"id":1,"name":"Ken","lastSeen":"2009-11-10T23:00:00Z"}

But what if we want to change how one of the field values are displayed? For example, say I wanted LastSeen to be a unix timestamp.

The simple solution is to introduce another auxiliary struct and populate it with the correctly formatted values in the MarshalJSON method.

func (u *MyUser) MarshalJSON() ([]byte, error) {
	return json.Marshal(&struct {
		ID       int64  `json:"id"`
		Name     string `json:"name"`
		LastSeen int64  `json:"lastSeen"`
	}{
		ID:       u.ID,
		Name:     u.Name,
		LastSeen: u.LastSeen.Unix(),
	})
}

This works, but it can get cumbersome when there are lots of fields. It would be nice if we could embed the original struct into the auxiliary struct and have it inherit all the fields that do not need to be changed.

func (u *MyUser) MarshalJSON() ([]byte, error) {
	return json.Marshal(&struct {
		LastSeen int64 `json:"lastSeen"`
		*MyUser
	}{
		LastSeen: u.LastSeen.Unix(),
		MyUser:   u,
	})
}

The problem here is that the auxiliary struct will also inherit the original’s MarshalJSON method, causing it to go into an infinite loop. The solution is to alias the original type. This alias will have all the same fields, but none of the methods.

func (u *MyUser) MarshalJSON() ([]byte, error) {
	type Alias MyUser
	return json.Marshal(&struct {
		LastSeen int64 `json:"lastSeen"`
		*Alias
	}{
		LastSeen: u.LastSeen.Unix(),
		Alias:    (*Alias)(u),
	})
}

The same technique can be used for implementing an UnmarshalJSON method.

func (u *MyUser) UnmarshalJSON(data []byte) error {
	type Alias MyUser
	aux := &struct {
		LastSeen int64 `json:"lastSeen"`
		*Alias
	}{
		Alias: (*Alias)(u),
	}
	if err := json.Unmarshal(data, &aux); err != nil {
		return err
	}
	u.LastSeen = time.Unix(aux.LastSeen, 0)
	return nil
}