如何使用NHibernate为我的更新命令添加其他条件？

I have situation where a Message table has a primary key (Id) and a foreign key (Site). Any site can create a message record, but only the site that created the record can update it. The problem is that by default it will be updated based upon Id and not Id and Site. I could change the primary key to be a composite/compound id consisting of the id and site OR resort to Native SQL; however, I was wondering if there is a way to add additional Update criteria.I have situation where a Message table has a pr