Rename struct's fields while keep supporting old JSON data

callback_test.go

package callback_test

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

type Data struct {
	// renamed from ClickTime
	EngagementTime time.Time `json:"engagement_time,omitempty"`

	// Deprecated: legacy field we want to switch from gracefully; use EngagementTime
	ClickTime time.Time `json:",omitempty"`
}

func (data *Data) UnmarshalJSON(b []byte) error {
	// new type to prevent recursive Data.UnmarshalJSON calls
	type NoMethodsData Data

	var legacyData NoMethodsData
	if err := json.Unmarshal(b, &legacyData); err != nil {
		return err
	}

	*data = Data(legacyData)
	if data.EngagementTime.IsZero() {
		// no new field was set, fallback to legacy field
		data.EngagementTime = legacyData.ClickTime
	}

	return nil
}

func TestData_legacyFields(t *testing.T) {
	t1 := time.Now().Truncate(time.Second)
	v1, _ := t1.MarshalText()

	cases := []struct {
		json string
		data Data
	}{
		{
			fmt.Sprintf(`{"engagement_time": "%s"}`, v1),
			Data{
				EngagementTime: t1,
				ClickTime:      time.Time{},
			},
		},
		{
			fmt.Sprintf(`{"ClickTime": "%s"}`, v1),
			Data{
				EngagementTime: t1,
				ClickTime:      t1,
			},
		},
	}

	for n, tc := range cases {
		var data Data
		if err := json.Unmarshal([]byte(tc.json), &data); err != nil {
			t.Fatal(err)
		}
		if !reflect.DeepEqual(tc.data, data) {
			t.Errorf("case %d: want %+v, got %+v", n, tc.data, data)
		}
	}
}