如何使用流畅的NHibernate将枚举映射为int值？

public enum UserRole
{
    Admin = 1,
    User = 2,
    Guest = 3
}

public class UserMap : ClassMap<User>
{
    public UserMap()
    {
        Id(x => x.Id);
        Map(x => x.Name);
        Map(x => x.Role).CustomType<UserRole>();
    }
}

using (var session = sessionFactory.OpenSession())
{
    using (var transaction = session.BeginTransaction())
    {
        var user = new User { Name = "John Doe", Role = UserRole.User };
        session.Save(user);
        transaction.Commit();
    }
}