[eluser]OverZealous[/eluser]
[quote author="KSiimson" date="1254361515"]Can I join a table which has a primary index that is not "id" and is not numerical? I would like to use a database table that is being used by other applications, so changing the primary index would break those apps.[/quote]
No, you cannot do this. Every DataMapper item must have a unique, auto-generated numerical ID.
The closest you can get to what you had before is to add an extra, DMZ primary key, and use your non-numerical ID as a different UNIQUEly indexed column.
The document you linked to is about managing complex relationships, but you still must use numerical indexes.