NHibernate.MappingException: No persister for: XYZ

Sounds like you forgot to add a mapping assembly to the session factory configuration..

If you're using app.config...

.
.
    <property name="show_sql">true</property>
    <property name="query.substitutions">true 1, false 0, yes 'Y', no 'N'</property>
    <mapping assembly="Project.DomainModel"/>  <!-- Here -->
</session-factory>
.
.

Something obvious, yet quite useful for someone new to NHibernate.

All XML Mapping files should be treated as Embedded Resources rather than the default Content. This option is set by editing the Build Action attribute in the file's properties.

XML files are then embedded into the assembly, and parsed at project startup during NHibernate's configuration phase.

My issue was that I forgot to put the .hbm in the name of the mapping xml. Also make sure you make it an embedded resource!

I got this off of here:

In my case the mapping class was not public. In other words, instead of:

public class UserMap : ClassMap<user>  // note the public!

I just had:

class UserMap : ClassMap<user>

Spending about 4 hours on googling and stackoverflowing, trying all of stuff around there, i've found my error:

My mapping file was called .nbm.xml instead of .hbm.xml. That was insane.