HABTM and collection select problem

I've been beating my head on this seemingly simple problem all evening
and would appreciate any pointers.

I have an Admin model and an Assistant model, both subclasses of User,
and with a HABTM relationship to each other (admins have many
assistants, and one assistant may work for multiple admins).

I have a join table called admin_assistant with columns of admin_id
and assistant_id.

In the form to create an Admin, I have:

<%= f.collection_select('assistant_id',
@assistants_list, :id, :name ) %>

While displays fine, but when the controller create action tries to
create the new Admin, I get:

Assistant expected, got String

The params have the ids for the selected assistants as an array of
strings:

"admin"=>{"name"=>"Joe Smith", "assistants"=>["2", "3"], ...}

I've googled all over for this and it seems most people get this
because the column name doesn't have _id, which I already have. But
none of the examples I could find used HABTM relationships -- does
this technique not work with the join table? What am I missing here?

Thanks for any pointers you can provide.

Michael

I found a way to get collection_select to work with a HABTM
relationship, by using the following for collection select:

<%= collection_select('user', 'assistant_ids',
@assistants_list, :id, :name, {}, {:multiple => true, :name
=>'user[assistant_ids][]' } ) %>

But it seems more complicated than it should be, and not well
documented. Am I missing a simpler way to do this? Or is really a rare
thing to be using collection_select with an HABTM relationship?

(In my case, it is a self-referential habtm relationship, but that
doesn't seem to be the issue. I abandoned the subclasses mentioned in
the description below for other reasons.)

Michael

P.S. Thanks to srjoseph who submitted this doc patch that gives this
solution: http://dev.rubyonrails.org/ticket/4837