Example of using Go json.RawMessage to unmarshal JSON field in struct based on another field determining its expected type.

twofactorunmarshal_test.go

package twofactorunmarshal

import (
	"encoding/json"
	"fmt"
	"testing"

	"github.com/pkg/errors"
)

type Event string

const (
	EventRegister Event = "register"
	EventOrder    Event = "order"
)

type RegisterData struct {
	Username string
	Password string
}

type OrderData struct {
	Currency string
	Amount   float64
}

type Message struct {
	Event Event
	Data  json.RawMessage
}

func handleMessage(m *Message) error {
	switch m.Event {
	case EventRegister:
		var data RegisterData
		if err := json.Unmarshal(m.Data, &data); err != nil {
			return errors.Wrap(err, "unmarshaling FormOnSubmitData")
		}
		return handleRegister(&data)
	case EventOrder:
		var data OrderData
		if err := json.Unmarshal(m.Data, &data); err != nil {
			return errors.Wrap(err, "unmarshaling InputOnChangeData")
		}
		return handleOrder(&data)
	default:
		return errors.Errorf("unknown event: %s", m.Event)
	}
}

func handleRegister(data *RegisterData) error {
	if data.Username == "" {
		return errors.New("username cannot be empty")
	}
	if len(data.Password) < 6 {
		return errors.New("password is too short")
	}
	fmt.Println("registered user!")
	return nil
}

func handleOrder(data *OrderData) error {
	if data.Currency == "" {
		return errors.New("currency cannot be empty")
	}
	if data.Amount < 0 {
		return errors.New("amount can not be negative")
	}
	fmt.Println("order created successfully!")
	return nil
}

func TestUnmarshal(t *testing.T) {
	t.Run("register", func(t *testing.T) {
		messageJSON := []byte(`{"event": "register", "data": {"username": "alex", "password": "123456"}}`)
		var m Message
		err := json.Unmarshal(messageJSON, &m)
		if err != nil {
			t.Fatal(err)
		}
		err = handleMessage(&m)
		if err != nil {
			t.Fatal(err)
		}
	})

	t.Run("order invalid data", func(t *testing.T) {
		messageJSON := []byte(`{"event": "order", "data": [1,2,3]}`)
		var m Message
		err := json.Unmarshal(messageJSON, &m)
		if err != nil {
			t.Fatal(err)
		}
		err = handleMessage(&m)
		if err == nil {
			t.Fatal("expected to fail because data is in unexpected structure")
		}
	})
}