Value objects in Go with private fields

valueobjects.go

package valueobjects

import (
	"encoding/json"
	"fmt"
)

type NodeIdentifier struct {
	identifier string
}

func NewNodeIdentifier(identifier string) (NodeIdentifier, error) {
	var v NodeIdentifier
	if err := v.setIdentifier(identifier); err != nil {
		return v, err
	}
	return v, nil
}

func (v *NodeIdentifier) setIdentifier(identifier string) error {
	if identifier == "" {
		return fmt.Errorf("Identifier must not be empty")
	}
	v.identifier = identifier
	return nil
}

func (v NodeIdentifier) Identifier() string {
	return v.identifier
}

func (v NodeIdentifier) MarshalJSON() ([]byte, error) {
	return json.Marshal(struct {
		Identifier string `json:"identifier"`
	}{
		Identifier: v.identifier,
	})
}

func (v *NodeIdentifier) UnmarshalJSON(b []byte) error {
	var d struct {
		Identifier string `json:"identifier"`
	}
	if err := json.Unmarshal(b, &d); err != nil {
		return err
	}
	if err := v.setIdentifier(d.Identifier); err != nil {
		return err
	}
	return nil
}

valueobjects_test.go

package valueobjects_test

import (
	"encoding/json"
	"testing"

	"github.com/stretchr/testify/assert"
	"networkteam.com/cqrs-test/valueobjects"
)

func TestNewNodeIdentifier_Empty(t *testing.T) {
	_, err := valueobjects.NewNodeIdentifier("")
	assert.Error(t, err)
}

func TestNewNodeIdentifier_Valid(t *testing.T) {
	v, err := valueobjects.NewNodeIdentifier("d20d4118-a28b-11e7-8f0d-777ee5502250")
	assert.NoError(t, err)
	assert.Equal(t, "d20d4118-a28b-11e7-8f0d-777ee5502250", v.Identifier())
}

func TestNodeIdentifier_JsonSerialize(t *testing.T) {
	v, err := valueobjects.NewNodeIdentifier("d20d4118-a28b-11e7-8f0d-777ee5502250")
	assert.NoError(t, err)

	b, err := json.Marshal(v)
	assert.NoError(t, err)

	assert.JSONEq(t, `{"identifier":"d20d4118-a28b-11e7-8f0d-777ee5502250"}`, string(b))
}

func TestNodeIdentifier_JsonUnserialize_Valid(t *testing.T) {
	var v valueobjects.NodeIdentifier
	var data = []byte(`{"identifier":"d20d4118-a28b-11e7-8f0d-777ee5502250"}`)
	err := json.Unmarshal(data, &v)
	assert.NoError(t, err)

	assert.Equal(t, "d20d4118-a28b-11e7-8f0d-777ee5502250", v.Identifier())
}

func TestNodeIdentifier_JsonUnserialize_Invalid(t *testing.T) {
	var v valueobjects.NodeIdentifier

	var data = []byte(`{"identifier":""}`)
	err := json.Unmarshal(data, &v)
	assert.Error(t, err)
}