class Account {
    private int id;
    private List<Favorite> favorites;
}

class Station {
    private int id;
    //...
}

class Favorite {
    private int id;
    private Account account;
    private Station station;
}

interface FavoriteRepository extends JpaRepository<Favorite, Integer> {
    public Favorite getOneByAccountAndStation(Account account, Station station);
}

class FavoriteService {
    private FavoriteRepository favoriteRepository;

    public newFav(Account a, Station s) {
        Favorite f = new Favorite(a, s);
        favoriteRepository.save(f);
        return f;
    }
}