Unsure how to model many-to-many relationship in MongoDB

Question

I'm new to MongoDB and I'm trying to model a Many to Many relationship into at least 2 collections (I need two collections for the project). What I'm having is a collection of universities, faculties and specializations, and another collection for students and their gradebook (this was the middle entity between specializations and students in SQL, not sure if it's needed in Mongo anymore). I tried to use This as an inspiration but it limits me as I can only search students by university id (I want for example to search students from a certain specialization or a certain faculty). I could put every row from university, faculty and specialization in the student collection and vice versa but I really don't think it's ideal. Here's what I have so far:

db.students.insertOne({_id:1, firstname: 'John', lastname: 'Silas', ethnicity:'english', civilstatus:'single', residence:'London', email:'johnSilas@gmail.com', gradebook:[{ year:2018, registrationyear:2017, formofeducation:'traditional'}], universities:[1]}) 

db.universities.insertOne({_id:1, name:'University of London', city:'London', adress:'whatever', phone: 'whatever', email: 'whatever@gmail.com', faculty:[{name: 'Law', adress:'whatever', phone: 'whatever', email: 'whatever@gmail.com'}], specialization:[{name:'criminal rights', yearlytax:5000, duration: 3, level:'bachelordegree', language:'english'}], students: [1,2]}) 

I'm sorry if I don't understand basic noSQL concepts, I am new to it. Thanks in advance.

Answer 1

Basic patterns for many to many association between A and B:

Inline references

• On A, store the list of B ids in a field like b_ids
• On B, store the list of A ids in a field like a_ids

This requires two writes whenever an association is created or destroyed, but requires either zero or one joins at query time to traverse the association (if you just want the id of Bs for a given A and you have the A already no further queries are needed).

Join model

Create a model C which has two fields: a_id and b_id . Each association is represented by a single instance of C.

This requires one write whenever an association is created or destroyed, but requires joins on all queries involving association (potentially two joins per query).