Composite primary keys and :joins=>

I have a legacy db with the following simplified structure:

   type_key, code_key, name, ... # PKs are type_key and code_key,
there is no id col and I cannot alter this db

Table-B: # each row has only the code as a foreign_key, the type_key
is hard-coded to "FOOKEY"
   an_id, code, ...

  class TableB < AR::Base
     set_table_name 'table_b'
     set_primary_key :an_id # singular PK

belongs_to :code_name, :class_name=>'TableA', :primary_key=>:pk_code_name, :foreign_key=>:code

     SELECT = "*, AS pk_name"

     def self.get_all
        all(:select=>SELECT, :joins=>:code_name)

  class TableA < AR::Base
     set_table_name 'table_a'
     set primary_keys :type_key, :code_key # using the composite
primary keys gem 2.3.2

     def self.find_by_pk_code_name(*args) # pk method invoked by
belongs_to in TableB
        opts = args.extract_options!
        pk_type_key = "FOOKEY"
        pk_code_key = args.first
        find([pk_type_key, pk_code_key], opts)
     rescue ActiveRecord::RecordNotFound

   x.code_name # works!
   TableB.get_all # fails: "element size differs (1 should be 2)"

I believe this error is due to 2 PKs being defined for TableA yet only
1 key is specified in the belongs_to :foreign_key.

The problem is that a row in TableB contains only one of the PKs for
TableA, with the second PK being hard-coded to "FOOKEY".
It appears that the contents of :foreign_key=> references col names,
not methods. The SQL I want to generate is something like this:
   SELECT *, AS pk_name FROM table_b
   LEFT OUTER JOIN table_a ON (table_a.code_key = table_b.code AND
table_a.type_key = 'FOOKEY')

I cannot figure out how to use AR to make this work. Any help is much
appreciated! Thanks!

as a follow up...I did the equivalent of:
   TableB.all(:select=>SELECT, :joins=>"left outer join table_a on
table_a.type_key='FOOKEY' and table_a.code_key=table_b.code")

and it works fine. Still curious to know if there is a way to define
a belongs_to and use that association in the :joins.