NHibernate审计-PostInsert不工作

我的应用程序具有以下实体:

public class User
{
    public virtual int UserID { get; set; }
    public virtual string UserName { get; set; }
    public virtual IList<UserLog> Log { get; private set; }

    public User()
    {
        Log = new List<UserLog>();
    }
}

public class UserLog
{
    public virtual int UserLogID { get; set; }
    public virtual User User { get; set; }
    public virtual string UserName { get; set; }
    public virtual DateTime DateCreated { get; set; }
}

public class UserMap : ClassMap<User>
{
    public UserMap()
    {
        Table("Users");
        Id(x => x.UserID);
        Map(x => x.UserName);
        HasMany(x => x.Log)
            .KeyColumn("UserID")
            .OrderBy("DateCreated")
            .Inverse()
            .Cascade.All();
    }
}

public class UserLogMap : ClassMap<UserLog>
{
    public UserLogMap()
    {
        Table("UsersLog");
        Id(x => x.UserLogID);
        References(x => x.User, "UserID");
        Map(x => x.UserName);
        Map(x => x.DateCreated);
    }
}

UsersLog表只记录对用户所做的任何更改。我正在尝试使用NHibernate事件侦听器自动连接这个。我已经设置了成功调用以下2个方法的配置:

public void OnPostInsert(PostInsertEvent @event)
{
    if (@event.Entity is User)
        InsertLog(@event.Entity);
}

public void OnPostUpdate(PostUpdateEvent @event)
{
    if (@event.Entity is User)
        InsertLog(@event.Entity);
}

编辑(这里是InsertLog方法):

private void InsertLog(object entity)
{
    var context = ServiceLocator.Current.GetInstance<IDataContext>();
    var user = (User)entity;
    context.Repository<UserLog>().Insert(new UserLog
    {
        User = user,
        UserName = user.UserName,
        DateCreated = DateTime.UtcNow
    }); // Insert calls ISession.SaveOrUpdate(entity)
}

当我更新一个记录时,一个日志被成功插入,但是当我插入一个记录时,我得到一个错误,告诉我UsersLog表中的UserID不能为null。我玩过一些不同的变化,但似乎没有什么工作。

如果有人能告诉我怎么做我会非常感激的。谢谢