How to organise a many to many relationship in MongoDB

What I've seen done, and what I currently use are embedded arrays with node id's in each document.

So document user1 has property groups: [id1,id2]

And document group1 has property users: [user1]. Document group2 also has property users: [user1].

This way you get a Group object and easily select all related users, and the same for the User.

This takes a bit more work when creating and updating the object. When you say 2 objects are related, you have to update both objects.

There's also a concept DBReferences in MongoDB and depending on your driver, it'll pull referenced objects automatically when retrieving a document.

http://www.mongodb.org/display/DOCS/Database+References#DatabaseReferences-DBRef

In-case anyone interested, I just bumped into a very good article posted in mongoDB blog. 6 Rules of Thumb for MongoDB Schema Design. There are 3 parts in this article, after reading all 3 you'll have a good understanding.

Let's understand Many to Many Relations with an examples

• books to authors
• students to teachers

The books to authors is a few to few relationship, so we can have either an array of books or authors inside another's document. Same goes for students to teachers. We could also embed at the risk of duplication. However this will required that each student has a teacher in the system before insertion and vice versa. The application logic may always not allow it. In other words, the parent object must exist for the child object to exist.

But when you have many to many relationship, use two collections and have a true linking.