DataMapper ORM v1.8.0

[eluser]WanWizard[/eluser]
Are you using the latest version? There should be no reference to OverZealous' website since I've taken over. Download the tip from http://bitbucket.org/wanwizard/datamapper to be sure. Or use the online user guide at http://datamappper.wanwizard.eu.

There is no default ordering, queries return data in whatever order your database will present them if you don't include an ORDER BY clause in your query.

Messaging is complex business, especially if you're buiding for scale. What I've done for a CMS I'm working on is to create the indices as nested trees (in my case forums, threads and PM's), and made sure the records are fixed length. The message text itself is in a separate table, linked to by the id, together with specific message attributes like author, timestamps, etc. This will also allow the message to appear in multiple places.

You can use Datamapper's nested sets extension, which works pretty ok for standard nested set constructions. It's documented in the user guide.