fleet.go

type FleetVehicle interface {
	MakeAndModel() string
}

type Vehicle struct {
	Type  string `json:"type"`
	Make  string `json:"make"`
	Model string `json:"model"`
}

func (v Vehicle) MakeAndModel() string {
	return fmt.Sprintf("%s %s", v.Make, v.Model)
}

type Car struct {
	Vehicle
	SeatingCapacity int     `json:"seatingCapacity"`
	TopSpeed        float64 `json:"topSpeed"`
}

type Truck struct {
	Vehicle
	PayloadCapacity float64 `json:"payloadCapacity"`
}

type Bike struct {
	Vehicle
	TopSpeed float64 `json:"topSpeed"`
}

type Fleet struct {
	Vehicles    []FleetVehicle    `json:"-"`
	RawVehicles []json.RawMessage `json:"vehicles"`
}

func (f *Fleet) MarshalJSON() ([]byte, error) {

	type fleet Fleet

	if f.Vehicles != nil {
		for _, v := range f.Vehicles {
			b, err := json.Marshal(v)
			if err != nil {
				return nil, err
			}
			f.RawVehicles = append(f.RawVehicles, b)
		}
	}

	return json.Marshal((*fleet)(f))
}

func (f *Fleet) UnmarshalJSON(b []byte) error {

	type fleet Fleet

	err := json.Unmarshal(b, (*fleet)(f))
	if err != nil {
		return err
	}

	for _, raw := range f.RawVehicles {

		var v Vehicle
		err = json.Unmarshal(raw, &v)
		if err != nil {
			return err
		}

		var i FleetVehicle
		switch v.Type {
		case "car":
			i = &Car{}
		case "truck":
			i = &Truck{}
		case "bike":
			i = &Bike{}
		default:
			return errors.New("unknown vehicle type")
		}

		err = json.Unmarshal(raw, i)
		if err != nil {
			return err
		}

		f.Vehicles = append(f.Vehicles, i)
	}

	return nil
}

fleet_test.go

import (
	"encoding/json"
	"testing"
)

func TestJSONMarshal(t *testing.T) {

	fleet := &Fleet{
		Vehicles: []FleetVehicle{
			&Car{Vehicle{"car", "Mercedes-Benz", "AMG C 63"}, 4, 250},
			&Bike{Vehicle{"bike", "Triumph", "Bonneville T120"}, 193.12},
		},
	}

	b, err := json.MarshalIndent(&fleet, "", "  ")
	if err != nil {
		t.Fatal(err)
	}

	t.Logf("%s\n", b)
}

func TestJSONUnmarshal(t *testing.T) {

	const body = `{
		"vehicles": [
			{"type": "car", "make": "BMW", "model": "M3", "seatingCapacity": 4, "topSpeed": 250}, 
			{"type": "truck", "make": "Volvo", "model": "FH", "payloadCapacity": 40000},
			{"type": "bike", "make": "Yamaha", "model": "YZF-R1", "topSpeed": 293}
		]
	}`

	var f *Fleet
	err := json.Unmarshal([]byte(body), &f)
	if err != nil {
		t.Fatal(err)
	}

	for _, vehicle := range f.Vehicles {

		if _, ok := vehicle.(FleetVehicle); !ok {
			t.Fatal("expected type to implement FleetVehicle")
		}

		switch v := vehicle.(type) {
		case *Car:
			t.Logf("%s has a seating capacity of %d and a top speed of %.2f km/h\n",
				v.MakeAndModel(),
				v.SeatingCapacity,
				v.TopSpeed)
		case *Truck:
			t.Logf("%s has a payload capacity of %.2f kg\n",
				v.MakeAndModel(),
				v.PayloadCapacity)
		case *Bike:
			t.Logf("%s has a top speed of %.2f km/h\n",
				v.MakeAndModel(),
				v.TopSpeed)
		}
	}
}

fleet_test_output.txt

=== RUN   TestJSONMarshal
--- PASS: TestJSONMarshal (0.00s)
    json_test.go:22: {
          "vehicles": [
            {
              "type": "car",
              "make": "Mercedes-Benz",
              "model": "AMG C 63",
              "seatingCapacity": 4,
              "topSpeed": 250
            },
            {
              "type": "bike",
              "make": "Triumph",
              "model": "Bonneville T120",
              "topSpeed": 193.12
            }
          ]
        }
=== RUN   TestJSONUnmarshal
--- PASS: TestJSONUnmarshal (0.00s)
    json_test.go:45: BMW M3 has a seating capacity of 4 and a top speed of 250.00 km/h
    json_test.go:50: Volvo FH has a payload capacity of 40000.00 kg
    json_test.go:54: Yamaha YZF-R1 has a top speed of 293.00 km/h
PASS

I'm new to Go, but is there any way to make this deserialisation logic reusable? E.g. could we add some custom tags to the vehicles to tell the name of discriminator column and subtype mapping?

Something like:

type Fleet struct {
    Vehicles []FleetVehicle `json:"vehicles" discriminator:"type,car=Car,truck=Truck,bike=Bike"`
}

And then have some magic which can look into these tags and instantiate?

Hi @inikolaev, as far as I'm aware there is no such functionality in encoding/json. My attempt was intended at replicating this functionality, but of course, it requires additional effort. One of the third-party JSON packages perhaps?

note that strictly speaking, RawVehicles []json.RawMessage `json:"vehicles"` does not need to be part of the Fleet type.
It can be local to the custom marshalling functions.