MongoDB數據庫設計準則

1.嵌入式文檔
如果需要在其它文檔上下文(父文檔)查看某個數據，則選擇把該數據嵌入指定上下文中。
例如：在應用中，地址信息通常不是單獨顯示而是隨着個人信息一起顯示，在這種場景之下應該採用嵌入式文檔。

{
   _id: "joe",
   name: "Joe Bookreader",
   addresses: [
                {
                  street: "123 Fake Street",
                  city: "Faketon",
                  state: "MA",
                  zip: "12345"
                },
                {
                  street: "1 Some Other Street",
                  city: "Boston",
                  state: "MA",
                  zip: "12345"
                }
              ]
 }

優點：檢索該文檔的全部信息只用一次查詢即可。

2.引用式文檔
採用時機：

1. 表現複雜的多對多關係
2. 文檔具有較大的層次數據集（一個文檔最大隻能存儲16M）
3. 嵌入式帶來數據冗餘，但是讀取效率又不能遠遠超過數據冗餘所帶來的犧牲

嵌入式
{
   title: "MongoDB: The Definitive Guide",
   author: [ "Kristina Chodorow", "Mike Dirolf" ],
   published_date: ISODate("2010-09-24"),
   pages: 216,
   language: "English",
   publisher: {
              name: "O'Reilly Media",
              founded: 1980,
              location: "CA"
            }
}

{
   title: "50 Tips and Tricks for MongoDB Developer",
   author: "Kristina Chodorow",
   published_date: ISODate("2011-05-06"),
   pages: 68,
   language: "English",
   publisher: {
              name: "O'Reilly Media",
              founded: 1980,
              location: "CA"
            }
}

上述模型造成了發佈者數據重複，爲了改善這種情況可採用引用式。引用式又分兩種情況：

1.多的一端只是許多，不是特多，有數量範圍的，則可以把引用放在發佈者一端

**publisher**
{
   name: "O'Reilly Media",
   founded: 1980,
   location: "CA",
   books: [123456789, 234567890, ...]
}

**book**
{
    _id: 123456789,
    title: "MongoDB: The Definitive Guide",
    author: [ "Kristina Chodorow", "Mike Dirolf" ],
    published_date: ISODate("2010-09-24"),
    pages: 216,
    language: "English"
}

{
   _id: 234567890,
   title: "50 Tips and Tricks for MongoDB Developer",
   author: "Kristina Chodorow",
   published_date: ISODate("2011-05-06"),
   pages: 68,
   language: "English"
}

2.多的一端，數量極大，無法估量。則將”one”端的引用放在”many”的一端

**publisher**
{
   _id: "oreilly",
   name: "O'Reilly Media",
   founded: 1980,
   location: "CA"
}
**book**
{
   _id: 123456789,
   title: "MongoDB: The Definitive Guide",
   author: [ "Kristina Chodorow", "Mike Dirolf" ],
   published_date: ISODate("2010-09-24"),
   pages: 216,
   language: "English",
   publisher_id: "oreilly"
}

{
   _id: 234567890,
   title: "50 Tips and Tricks for MongoDB Developer",
   author: "Kristina Chodorow",
   published_date: ISODate("2011-05-06"),
   pages: 68,
   language: "English",
   publisher_id: "oreilly"
}