curl_cmd

curl -X POST localhost:8888/check-in -d '{"pet-owner":{"pet":{"Type":"dog","Value":"{\"Name\":\"POCHI\",\"Breed\":\"AKITA\"}"}}}'
{"pet":{"Type":"dog","Value":"{\"Name\":\"POCHI\",\"Breed\":\"AKITA\"}"}}

design.go

package design

import (
	. "goa.design/goa/v3/dsl"
)

var Dog = Type("Dog", func() {
	Description("Dogs are cool")
	Field(1, "Name")
	Field(2, "Breed")
	Required("Name", "Breed")
})

var Cat = Type("Cat", func() {
	Description("Cats are cool too")
	Field(1, "Name")
	Field(2, "Age", Int)
	Required("Name", "Age")
})

var PetOwner = Type("PetOwner", func() {
	Description("A pet owner")
	OneOf("pet", func() {
		Description("The owner's pet")
		Field(1, "dog", Dog)
		Field(2, "cat", Cat)
	})
})

var _ = Service("pet-hotel", func() {
	Method("check-in", func() {
		Payload(func() {
			// Attribute describes an object field
			Attribute("pet-owner", PetOwner, func() {
				Meta("rpc:tag", "1")
			})
			Required("pet-owner")
		})
		Result(PetOwner)
		HTTP(func() {
			POST("/check-in")
			Response(StatusOK)
		})
		GRPC(func() {
			Response(CodeOK)
		})
	})
})

pet_hotel.go

package pethotelapi

import (
	"context"
	"fmt"
	"log"

	pethotel "goa.design/examples/union/gen/pet_hotel"
)

// pet-hotel service example implementation.
// The example methods log the requests and return zero values.
type petHotelsrvc struct {
	logger *log.Logger
}

// NewPetHotel returns the pet-hotel service implementation.
func NewPetHotel(logger *log.Logger) pethotel.Service {
	return &petHotelsrvc{logger}
}

// CheckIn implements check-in.
func (s *petHotelsrvc) CheckIn(ctx context.Context, p *pethotel.CheckInPayload) (*pethotel.PetOwner, error) {
	s.logger.Print("petHotel.check-in")
	switch p.PetOwner.Pet.(type) {
	case *pethotel.Dog:
		fmt.Println("bow bow")
	case *pethotel.Cat:
		fmt.Println("mew mew")
	default:
		return nil, fmt.Errorf("unknown type: %T", p.PetOwner.Pet)
	}
	res := &pethotel.PetOwner{
		Pet: p.PetOwner.Pet,
	}
	return res, nil
}