[Zope] Archetypes fields in an associative link?

[Bryan Simmons]

Suppose I want 2 fields of an Archetypes product to be read-only to
users and populated based on the value of a 3rd field?

I want users to choose a project and have the charge number for the
project and project manager to be filled automagically.  My problem is
that I can't tell what Archetypes field/widget would be best for this
job.

I was thinking that the 2 dependant fields could be bound to a method
to get their values, but what type of field will hold a key and a
bunch of associated values that can be altered in a form via some sort
of widget?

I'm relatively new to Archetypes.  From what I've seen, this
functionality is sure to exist already.  I'm perfectly willing to
write a custom field or custom widget for this, I just want to make
sure I'm not reinventing the wheel.
-- 

Regards,

Bryan Simmons

  "All parts should go together without forcing. You must remember
that the parts you are reassembling were disassembled by you.
Therefore, if you can't get them together again, there must be a
reason. By all means, do not use a hammer."
  -- IBM maintenance manual, 1925