legacy database, primary key compound with two fields

I am trying to define a model for a legacy database and the primary key
is the combination of two fields, 'branchcode' and 'debtorno'

class Custbranch < ActiveRecord::Base

  set_table_name 'custbranch'
  set_primary_key 'branchcode_branchno'

end

and I have tried...

  set_primary_key 'branchcode branchno'
  set_primary_key 'branchcode, branchno'

but in the console, a simple find, always an error...

@branch=Custbranch.find("A365")

ActiveRecord::StatementInvalid: Mysql::Error: Unknown column
'custbranch.branchcode_branchno' in 'where clause': SELECT * FROM
`custbranch` WHERE (`custbranch`.`branchcode_branchno` = 'A365')

@branch = Custbranch.find("A365")

ActiveRecord::StatementInvalid: Mysql::Error: Unknown column
'custbranch.branchcode branchno' in 'where clause': SELECT * FROM
`custbranch` WHERE (`custbranch`.`branchcode branchno` = 'A365')

@branch = Custbranch.find("A365")

ActiveRecord::StatementInvalid: Mysql::Error: Unknown column
'custbranch.branchcode, branchno' in 'where clause': SELECT * FROM
`custbranch` WHERE (`custbranch`.`branchcode, branchno` = 'A365')

and the only way I seem to make it work is to use 'branchcode' as the
primary key which is not indexed. I can use this for now but I am
wondering if it is possible to set_primary_key on a legacy db model to
use a compound index/primary key.

Craig

Craig White wrote:

I am trying to define a model for a legacy database and the primary key
is the combination of two fields, 'branchcode' and 'debtorno'

[...]

If you can't add a surrogate key, try the composite_primary_keys plugin.

Best,