"【Question】 Imagine you have a collection for posts, and each of these posts has the attribute us .."

blackduckie RaqForum 28 No.
604 View • 4 Years ago

How Do I Join Collections in MongoDB?

SQL(179)

【Question】

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?

【Answer】

MongoDB does not support joins and so requires repeated queries over respectively over the two collections. It would be convenient if we could handle the join separately in a computing engine, such as Structured Process Language (SPL). Suppose I have two collections – employee and seller and need to find sales orders where the employee.state is California. The SPL script is as below:

	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)