キーの名前にルールはあるけど一定じゃないキーがあるJSONをデコードするやつ

variable_key_json.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"strings"
)

type Payload struct {
	PayloadContents
	Sites map[string]Site `json:"-"`
}

type PayloadContents struct {
	ID string
}

type Site struct {
	Name string
}

func (p *Payload) UnmarshalJSON(b []byte) error {
	if err := json.Unmarshal(b, &p.PayloadContents); err != nil {
		return fmt.Errorf("failed to decode contents", err)
	}

	rawSites := map[string]json.RawMessage{}
	if err := json.Unmarshal(b, &rawSites); err != nil {
		return fmt.Errorf("failed to decode sites", err)
	}

	p.Sites = map[string]Site{}
	for k, v := range rawSites {
		if !strings.HasPrefix(k, "https://") {
			continue
		}

		site := Site{}
		if err := json.Unmarshal(v, &site); err != nil {
			return fmt.Errorf("failed to decode site %q: %w", k, err)
		}
		p.Sites[k] = site
	}

	return nil
}

func main() {
	jsonText := `
{
  "https://www.google.com": {"name": "google"},
  "https://www.mozila.org": {"name": "mozilla"},
  "id": "moo"
}`
	payload := Payload{}
	if err := json.Unmarshal([]byte(jsonText), &payload); err != nil {
		log.Fatalf("failed do decode paylaod: %+v", err)
	}

	fmt.Printf("%+v", payload)
}