Custom UnmarshalJSON implementation to support a single property with string/object values

unmarshal.go

package main

import (
	"encoding/json"
)

type Business struct {
	ID   string `json:"id"`
	Name string `json:"name"`
}

type BusinessRef struct {
	Business
	IDOnly bool
}

func (bj *BusinessRef) UnmarshalJSON(data []byte) error {
	var id string
	err := json.Unmarshal(data, &id)
	if err != nil {
		// if the err was because the value found is an object,
		// try again with the Business struct
		typeErr, ok := err.(*json.UnmarshalTypeError)
		if ok && typeErr.Value == "object" {
			return json.Unmarshal(data, &bj.Business)
		}
		// otherwise, return the original error
		return err
	}
	bj.ID = id
	bj.IDOnly = true // indicate that business was just a string
	return nil
}

type JobOffer struct {
	Business *BusinessRef `json:"business"`
	Position string       `json:"position"`
}

unmarshal_test.go

package main

import (
	"encoding/json"
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestBusinessRef_UnmarshalJSON(t *testing.T) {
	t.Parallel()
	data := []byte(`{
	"business": "76261649-1",
	"position": "Software Developer"
}`)
	var jobOffer JobOffer
	err := json.Unmarshal(data, &jobOffer)
	assert.Nil(t, err)
	assert.Equal(t, "76261649-1", jobOffer.Business.ID)
	assert.True(t, jobOffer.Business.IDOnly)
	assert.Equal(t, "Software Developer", jobOffer.Position)
}

func TestBusinessRef_UnmarshalJSON2(t *testing.T) {
	t.Parallel()
	data := []byte(`{
	"business":{
		"id": "76261649-1",
		"name": "Larix Sistemas Informáticos Limitada"
	},
	"position": "Software Developer"
}`)
	var jobOffer JobOffer
	err := json.Unmarshal(data, &jobOffer)
	assert.Nil(t, err)
	assert.Equal(t, "76261649-1", jobOffer.Business.ID)
	assert.Equal(t, "Larix Sistemas Informáticos Limitada", jobOffer.Business.Name)
	assert.False(t, jobOffer.Business.IDOnly)
	assert.Equal(t, "Software Developer", jobOffer.Position)
}