Rails - May 2005 - single quotes around string primary keys using habtm and MySQL

I''m converting an application to rails using an existing schema and
I''ve got a
problem with has_and_belongs_to_many. The two related tables have a string
primary key and whenever I add a new item with things.other_things.push or
push_with_attributes the ID entries in the joining table have single quotes
around them. i.e. joining BANK1 to BROKER1 gives me entries with
''BANK1'' and
''BROKER1''. Next time round rails doesn''t think that
there''s a relationship
between BANK1 and BROKER1. But if I manually update the row to remove the
single quotes then
everything works fine (at least until I add a new relationship).

Does anyone have any ideas?
Thanks
Chris