belongs_to primary key

I’m writing a rails application that uses a
legacy database that is imported. The legacy database has primary keys
like “appno”. The new structure in rails is going to be “id,
company_id, appno” so there may be more than one record with the same
appno, but each having a unique company_id, appno pair. id will be
completely unique.

The joined record will have the structure “id, company_id, jobno,
appno” where company_id and jobno form a unique pair, and id is unique.
I want to say that the job belongs_to :applicant, :foreign_key =>
“appno”, :primary_key => “appno”, :conditions => [“company_id =
?”, self.company_id] so that it won’t use the id field on the applicant
record but will instead use the appno field. Is it possible to do this?