What is the recommended equivalent of cascaded delete in MongoDB for N:M relationships?

Question

Assuming the following \"schema/relationship\" design what is the recommended practice for handling deletion with cascade delete like operation?

Relational Schema:

Answer 1

I'm going to answer based on Mongo team recommendations. I also came from the relational database and I had some issues at the beginning understanding the concepts. Mongo team recommends to design with the idea of "Application-Driven" schema, so you have to figure out first what pieces of data go together. Remember there's not such a transaction concept in any possible way in Mongo, even if we invent a driver that handles transactions we should implement our own solution for this. It means if I have two business objects that requires to be updated at the same time always and I cannot tolerate a failure in this operation, I have to join them into a single document (atomic).

In your case you have two documents, Student and Courses, and a relation between then (A student enrolls to N courses). I assume courses are not required to be altered all the time, so they can be stored in a different collection. But the point is the relation between them, in this case you need to atomically delete a Student and all the courses he enrolled in. So the best suitable solution for this is to embed the relation into Student, and keep a separated Course collection. When you delete the student, the relation is dropped at the same time:

Student Json:

{ _id: ObjectId('...'), name:"John", lastname:"Smith", 
courses: [ 1, 100, 50, 67 ], ...
}

Courses can be a separated collection between them. This is the way to handle it in Mongo. Atomic operations must be embedded into a single document. I assumed Courses is a list of courses that don't change so much, in case they're designed by Student we could change a bit the solution.