How to map property to first of many in Fluent NHibernate?

Question

Given a parent child relationship between User and FailedLogin where a user has many failed logins. I'd like to map this into

class User
{
    public virtual FailedLogin LastFailedLogin { get; set; }
}

class FailedLogin
{
    public virtual User User { get; set; }
    public virtual DateTime AttemptOn { get; set; }
}

So that the LastFailedLogin property contains the FailedLogin with the most recent AttemptOn date and time. If LastFailedLogin is set it should save that FailedLogin to the database (i.e. .Cascade.SaveUpdate()) Note, I do not want to map a collection of all FailedLogins for this user for performance reasons.

I have been totally unable to write a fluent mapping for this. How do I map this using Fluent NHibernate?

public class UserMap : ClassMap<User>
{
    public UserMap()
    {
        // What goes here for LastFailedLogin?
    }
}

public class FailedLoginMap: ClassMap<FailedLogin>
{
    public FailedLoginMap()
    {
        References(x => x.User).Not.Update();
        Map(x => x.AttemptOn).Not.Update();
    }
}