c# /NHibernate:关联引用未映射的类

I have been struggling with this NHibernate issue for hours. I extensively researched on the web as well as the NHibernate documentation, and I can make no sense of this issue. I'm relatively new to NHibernate, and love it. In that case, though, it's driving me mad.I have been struggling with this NHibernate iss