Test.js

db.createCollection("Folders");

db.Folders.ensureIndex({ "UserId": 1 });

db.Folders.insert({ "UserId": 1 });

db.Folders.find();

db.Folders.update({ "UserId": 1 }, { $set: { "Folders.A": { "Name": "A" } } });

db.Folders.find();

db.Folders.update({ "UserId": 1 }, { $set: { "Folders.B": { "Name": "B" } } });

db.Folders.find();

db.Folders.update({ "UserId": 1 }, { $unset: { "Folders.B" : "" } });

db.Folders.find();

db.Folders.update({ "UserId": 1}, { $set: { "Folders.A.Folders.A": { "Name": "A" } } });

db.Folders.find();