Rails - May 2005 - Form select helpers for m-to-n and m-to-m

I think this already has been asked about, but still.

How do I easily generate a select-multiple and select form element   
(or a checkbox array) for an ActiveRecord that has a m-t-m or m-to-n  
relationship? Of course I would love to have the existing relations  
already selected etc.

In my current app I have a Work, which has 1 client and many Activities.

I use this for the select of the client:

<%= collection_select ''work'', ''client'',
Client.find_all, :id, :name,
@work.client.id  %>

This gives me an empty option tag in the beginning of the form when  
editing. How can I pump the client ID which already is set for the  
record for an edit action and an empty option for create action? I am  
sure this is covered in one fo the tutorials but was changed in one  
of the recent verisons. The Rdocs are also not so clear on that one.

-- 
Julian "Julik" Tarkhanov