DataMapper tertiary relations

[eluser]Dracos[/eluser]
Previously I've asked about handling the relations between my app and user models, now there's a third one in the mix. (DM 1.8)

Our apps_users table has some extra fields:

Code:

id
app_id
user_id
contact_id
role
status

The contact_id field is supposed to point to the contact for the user that the app maintains. I've previously made a app_users model to bridge the gap, but I was wondering if there was a way to set up a relation so we could navigate from app to contact via apps_users. This is an edge case that I'm not seeing covered int he DM docs.