Implement MarshalJSON for type alias.

marshal.go

package main

import (
	"encoding/json"
	"fmt"
)

type Metadata struct {
	UUID string
	Name string
}

type Collection struct {
	Name     string
	Metadata Metadata
}

type AliasedMetadata Metadata

type AliasedCollection struct {
	Name     string
	Metadata AliasedMetadata
}

func (m Metadata) MarshalJSON() ([]byte, error) {
	return []byte(fmt.Sprintf("\"%s\"", m.UUID)), nil
}

func (m AliasedMetadata) MarshalJSON() ([]byte, error) {
	return []byte(fmt.Sprintf("\"%s-%s\"", m.Name, m.UUID)), nil
}

func toJSON(col interface{}) {
	out, err := json.Marshal(col)

	if err != nil {
		panic(err)
	}

	fmt.Println(string(out))
}

func main() {
	col := Collection{
		Name:     "MyCollection",
		Metadata: Metadata{UUID: "12345", Name: "MyMetadata"},
	}
	toJSON(col)

	col2 := AliasedCollection{
		Name:     "MyCollection",
		Metadata: AliasedMetadata{UUID: "12345", Name: "MyAliasedMetadata"},
	}
	toJSON(col2)
}