unable to create record with has_mant :through relationship

hi all,

i have a has_many :through r'ship in my application. my tables are -
users, apps(short form for applications), categorizations. i'm trying to
create a user in new.rhtml where all apps are also shown in checkboxes
for selection. on submit, user gets created but the join table i.e.
categorizations table doesn't get updated. where am i going wrong?
doesn't the join table gets updated automatically? or do i've to update
it manually? if yes, how? Any help will be greatly appreciated. thanks
in advance.

I beleive that you need to create the categorizations yourself
something like (this is untested):

class UsersController < ApplicationController
def new
@user = User.new
@apps = App.find(:all)

def create
@user = User.new(params[:user])

      params[:user][:app_ids].each do | app_id|
           @user.categorizations << Category.new(:app_id => app_id)

redirect_to(:action => "new")
flash[:notice] = "Thanks for signing up!"
rescue ActiveRecord::RecordInvalid
render :action => 'new'