@Document()


public class ToDo {
@Id
private Object id;
private List<Item> day;
private List<Item> week;
private List<Item> month;
private List<Item> year;
private List<Item> other;

ToDo(){
    this.id = new ObjectId();
    this.day = new ArrayList<>();
    this.week = new ArrayList<>();
    this.month = new ArrayList<>();
    this.year = new ArrayList<>();
    this.other = new ArrayList<>();
}

private class Item {

    @Id
    private Object id;
    private boolean completed;
    private String description;
    private Date date;

    public Item(String description, boolean completed) {
        this.description = description;
        this.completed = completed;
        this.date = new Date();
        this.id = new ObjectId(date);
    }
}

public void createItem(String name){
    new Item(name, false);
}