Rails - May 2005 - Please help a newbie: form helpers while iterating through a list

Hi all,

I''m having real problems using form helpers for complex relationships.

I have a table of users, and a table of subjects. A third table, 
''interests'', comprises the following fields: id, user_id,
subject_id,
description.

User has many interests; subjects has many interests; interest belongs 
to user and belongs to subject. So far so good.

Now, for various reasons, the ''edit interests'' view has to be
a single
page. So I need to find a way to iterate through @user.interests, 
producing a <textarea> for each subject. I''m having two problems
with
this:

1) I can''t figure out how to get the interest.id into the form helper. 
What should the text_area helper look like?
2) What do I do about the fields that are empty? Let''s say that the 
user has already entered an interest for one subject, but not the 
others. When s/he enters interests for some of the other subjects, I 
will have a variety of ''new'' entries to contend with. So I
suppose I
also need to get the department_id into the form helper too.

I tried adapting something like this:
http://one.textdrive.com/pipermail/rails/2005-January/001888.html
But without success.

Please can someone help?

Rob