fn main() {
    let mut session = UoW::new();
    let entry = session.load::<SomeEntity>();
    println!("{:?}", entry);
    entry.value().msg = "Changed".to_string();
    println!("{:?}", entry);

    let other_entry = session.load::<SomeOtherEntity>();
    other_entry.value().msg = "Other has also changed".to_string();

    session.commit();
}

use std::any::{Any, TypeId};
use std::marker::PhantomData;
#[derive(Debug)]
struct UoW {
    tracked: Vec<(TypeId, Box<Any>)>,
}

impl<'a> UoW {
    fn new() -> Self {
        UoW { tracked: vec![] }
    }

    fn load<T: Any + Entity + 'a>(&mut self) -> &mut Tracked<T> {
        let t = TypeId::of::<T>();
        let index = self.tracked.len();

        // Let's pretend we get from the database
        let repo = SomeRepo::new();
        let entry: T = repo.get_by_id(1);

        self.tracked
            .push((t, Box::new(Tracked::new(index, entry, State::Dirty))));
        let y = &mut self.tracked[index].1;
        // should be safe to unwrap since we know it's of type T
        y.downcast_mut::<Tracked<T>>().unwrap()
    }

    fn commit(&mut self) {
        let _some_entity_type = TypeId::of::<SomeEntity>();
        let _some_other_entity_type = TypeId::of::<SomeOtherEntity>();

        // -------- START TRANSACTION --------
        for (key, entry) in self.tracked.drain(..) {
            if key == _some_entity_type {
                  let tracked = entry.downcast_ref::<Tracked<SomeEntity>>().unwrap();
                println!(
                    "The frist entry will be \"{}\" to db: \n{:?}",
                    if *tracked.state() == State::Added {"added"} else {"updated"},
                    tracked
                );
            } else if key == _some_other_entity_type {
                // still, we know the type...
                let tracked = entry.downcast_ref::<Tracked<SomeOtherEntity>>().unwrap();
                println!(
                    "The other entry will be \"{}\" to db: \n{:?}",
                    if *tracked.state() == State::Added {"added"} else {"updated"},
                    tracked
                );
            }
        }
        // ------- COMMIT TRANSACTION --------
    }
}

#[derive(Debug)]
struct Tracked<T> {
    id: usize,
    inner: T,
    state: State,
}

impl<T> Tracked<T> {
    fn new(id: usize, inner: T, state: State) -> Self {
        Tracked { id, inner, state }
    }

    fn value(&mut self) -> &mut T {
        &mut self.inner
    }

    fn state(&self) -> &State {
        &self.state
    }
}

#[derive(Debug)]
struct SomeRepo<T> {
    _marker: PhantomData<T>,
}

impl<T: Entity> SomeRepo<T> {
    fn new() -> Self {
        SomeRepo {
            _marker: PhantomData,
        }
    }
    fn get_by_id(&self, id: i32) -> T {
        // get from db
        T::new(id)
    }
}

trait Entity {
    fn new(id: i32) -> Self;
}

#[derive(Debug, PartialEq)]
enum State {
    Added,
    Dirty,
    //...
}

#[derive(Debug)]
struct SomeEntity {
    pub id: i32,
    pub msg: String,
}

impl Entity for SomeEntity {
    fn new(id: i32) -> Self {
        SomeEntity {
            id,
            msg: String::from("Hello"),
        }
    }
}

#[derive(Debug)]
struct SomeOtherEntity {
    pub id: i32,
    pub msg: String,
}

impl Entity for SomeOtherEntity {
    fn new(id: i32) -> Self {
        SomeOtherEntity {
            id,
            msg: String::from("Other hello"),
        }
    }
}