Not sure how feasible this is, but working with relational data in MongoDB is always a pain since finding nested documents requires writing an aggregation using $lookup, $unwind, etc. 

Just thought of a convenient implementation to solve this. If only ObjectIds included a collection id, then you could call a "deep find" method which fetches all nested objects where it encounters an ObjectId in the return documents. 

Other considerations here would be infinite loop protection and a max depth feature.