mongodb 中做 join 的方法
【问题】
Imagine you have a collection for posts, and each of these posts has the attribute userid: ObjectId( ), where ObjectID is referencing a document in the Users collection.
How would you go about retrieving the user information (in this case, the user document might contain a username, email, full name, etc) as part of a query on the Posts collection?
【回答】
用 Mongodb 是不支持 join 查询的, 要多次查询很不方便。可以考虑配合 mongodb 使用的本身有计算能力的工具，比如使用 SPL。举例说明：
通过 employee 文档和 seller 文档查询 employee.state 为 California 的 sales orders。
| A | |
|---|---|
| 1 | =mongo_open(“mongo://localhost:27017/local?user=test&password=test”) |
| 2 | >sales=mongo_shell(A1,”test38.find()”) |
| 3 | >employee=mongo_shell(A1,”test39.find()”).switch(SELLERID,sales:EID) |
| 4 | =A3.select(SELLERID.STATE==“California”) |
| 5 | >mongo_close(A1) |
A2：sales
A3：employee
A4：选出满足条件的结果，订单 1、3 都属于来自 California 的雇员 1
