Database.js

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

db.Folders.insert({ "UserId": 1 });
db.Folders.update({ "UserId": 1 }, { $set: { "Folders.A": { "Name": "A" } } });
db.Folders.update({ "UserId": 1 }, { $set: { "Folders.B": { "Name": "B" } } });
db.Folders.update({ "UserId": 1 }, { $set: { "Folders.C": { "Name": "C" } } });
db.Folders.update({ "UserId": 1 }, { $set: { "Folders.D": { "Name": "D" } } });
db.Folders.update({ "UserId": 1 }, { $unset: { "Folders.B" : "" } });
db.Folders.update({ "UserId": 1}, { $set: { "Folders.A.Folders.A": { "Name": "A" } } });
db.Folders.find({ "UserId": 1 }, { "Folders.A.Folders.A": 1 });

Result.js

{
    "Folders" : {
        "A" : {
            "Folders" : {
                "A" : {
                    "Name" : "A"
                }
            }
        }
    },
    "_id" : ObjectId("525bbf0a53adb840d85647d3")
}