https://blog.toright.com/posts/4537/mongodb-schema-%e8%a8%ad%e8%a8%88%e6%8c%87%e5%8d%97-part-ii-%e5%8f%8d%e6%ad%a3%e8%a6%8f%e5%8c%96%e7%9a%84%e5%a8%81%e5%8a%9b
MongoDB Schema 設計指南 (Part II) - 反正規化的威力