Has many association problem

You didn't say if it exists, but I'm guessing that there's an
'encounter group' table associated to that number (99999). In which
case, what you're looking at is a has_many :through relation.

(code example simplified - fix up with modules, etc as your app needs)

class Patient < ActiveRecord::Base
  # has field pat_encounter_group
  belongs_to :encounter_group, :foreign_key => 'pat_encounter_group'
  has_many :requests, :through => :encounter_group
end

class EncounterGroup < ActiveRecord::Base
  has_many :patients, :foreign_key => 'pat_encounter_group'
  has_many :requests, :foreign_key => 'req_encounter_group'
end

class Request < ActiveRecord::Base
  belongs_to :encounter_group, :foreign_key => 'req_encounter_group'
  has_many :patients, :through => :encounter_group
end

[Note: I haven't tried this code, but it should work according to the
documentation]

On the other hand, the code you've got below should, in principle,
also work. How exactly did it "fail"?

--Matt Jones