I tried this :

select p from btweb.domainModel.Project p order by p.Client.Name

and the results are not sorted by p.Client.Name

if I try

select p.Client.Name from btweb.domainModel.Project p order by p.Client.Name

then the results are correctly sorted.

Client is declared in the Project.hbm.xml file as :

<many-to-one name="Client" class="btweb.domainModel.Client, btweb" column="clientId"/>

The Client has no reference to the Project class.

Is this a bug, or can I not sort by child a child (composite) object ?

Looks like a bug to me, can you please create a JIRA issue with a small example? Also, try looking what SQL NHibernate generates for the buggy query (either in your database server logs, or by enabling NHibernate logging).

Added issue NH-345

Cheers
Grant