DMZ 1.7.1 (DataMapper OverZealous Edition)

[eluser]WanWizard[/eluser]
If the relation is one-to-one-or-zero, why define it as has_many? Include_related only works on has_one.

You can find Datamapper's issue tracker on http://bitbucket.org/wanwizard/datamapper/issues.

As for the join_field, I understand the issue. If you just want the highest position, you could try order_by('position', 'DESC')->limit(1) to get only that record.