MongoDB, relation between documents or collections - mongodb

Question

I'm pretty new with NoSQL, MongoDB. How to deal with the many-to-many relation between 2 or multiple collections/documents? we'd better use DBRefs or embed? actually I've already read the MongoDB manual, but I didn't find something about many-to-many relation. I missed some points? or there is no this kind of relation in MongoDB? thx!

Answer 1

Embed versus reference
This is the problem of embedding versus referencing, and it’s a common source of confusion for new users of MongoDB. There’s a simple rule of thumb that works for most schema design scenarios: Embed when the child objects always appear in the context of their parent. Otherwise, store the child objects in a separate collection.
Embed or reference it depends on the application. Suppose you’re building a simple application in MongoDB that stores blog posts and comments.If the comments always appear within a blog post, and if they don’t need to be ordered in arbitrary ways (by post date, comment rank, and so on), then embedding is fine. But if, say, you want to be able to display the most recent comments, regardless of which post they appear on, then you’ll want to reference. Embedding may provide a slight performance advantage, but referencing is far more flexible.
Many-to-many
In RDBMSs, you use a join table to represent many-to-many relationships; in MongoDB, you use array keys. For example each product contains an array of category IDs, and both products and categories get their own collections. If you have two simple category documents
{ _id: ObjectId("4d6574baa6b804ea563c132a"),
title: "Epiphytes"
}
{ _id: ObjectId("4d6574baa6b804ea563c459d"),
title: "Greenhouse flowers"
}
then a product belonging to both categories will look like this:
{ _id: ObjectId("4d6574baa6b804ea563ca982"),
name: "Dragon Orchid",
category_ids: [ ObjectId("4d6574baa6b804ea563c132a"),
ObjectId("4d6574baa6b804ea563c459d") ]
}