Many to many update in MongoDB without transactions

Question

I have two collections with a many-to-many relationship. I want to store an array of linked ObjectIds in both documents so that I can take Document A and retrieve all linked Doc

Answer 1

Why don't you create a dedicated collection holding the relations between A and B as dedicated rows/documents as one would do it in a RDBMS. You can modify the relation table with one operation which is of course atomic.