Nested structures with quick-access index in Rust

use std::collections::HashMap;

#[derive(Debug)]
#[allow(dead_code)]
struct Entry {
    id: u64,
    title: String,
    username: String,
    password: String,
}

#[derive(Debug)]
#[allow(dead_code)]
struct Group {
    name: String,
    groups: Vec<Group>,
    entries: Vec<Entry>,
}

#[derive(Debug)]
struct Database<'a> {
    id_map: HashMap<u64, &'a Entry>,
    top: Group,
}

fn main() {
    let mut db = Database {
        top: Group {
            name: "Top-level".to_string(),
            groups: vec![
                Group {
                    name: "Games".to_string(),
                    groups: vec![],
                    entries: vec![
                        Entry {
                            id: 23,
                            title: "World of Warcraft".to_string(),
                            username: "darkelf123".to_string(),
                            password: "s3cr!t".to_string(),
                        },
                    ],
                },
            ],
            entries: vec![],
        },
        id_map: HashMap::new(),
    };

    db.id_map.insert(db.top.groups[0].entries[0].id, &db.top.groups[0].entries[0]);

    println!("Entry ID 23: {:#?}", db.id_map.get(&23));

    // Can no longer directly mutate entry:
    // let entry = &mut db.top.groups[0].entries[0];

    let mut entry = db.id_map.get_mut(&23).expect("Entry missing");
    // Can't modify entry through ID mapping
    // entry.username = "DarkElf123".to_string();
}