如何使用NHibernate插入或更新（或覆盖）记录？

I need to write a row to the database regardless of whether it already exists or not. Before using NHibernate this was done with a stored procedure. The procedure would attempt an update and if no rows were modified it would fallback to an insert. This worked well because the application doesn't care if the record exists.I need to write a row to the database regardles