automatic update with has_and_belongs_to_many (:through)

Hi,

I have an application with a relation (many to many) via a :through. I want this to be automatically updated just like with a regular many_to_many. Just like I read here: Peak Obsession

Is this possible? I just can't get it to work...

Here are my models:

class Declaration < ActiveRecord::Base   #Foreign key is niet nodig, maar wordt wel aangeraden   belongs_to :owner, :class_name => 'User', :foreign_key => 'user_id'   has_many :declarations_payers   has_many :users, :through => :declarations_payers

  validates_presence_of :user_id   validates_numericality_of :user_id   validates_presence_of :amount   validates_numericality_of :amount   validates_presence_of :description

  validates_associated :owner   validates_associated :users

end

class DeclarationsPayer < ActiveRecord::Base   belongs_to :user   belongs_to :declaration

  validates_presence_of :declaration_id   validates_numericality_of :declaration_id   validates_presence_of :user_id   validates_numericality_of :declaration_id

  validates_associated :user   validates_associated :declaration end

class User < ActiveRecord::Base   has_many :deposits   has_many :declarations_users   has_many :declarations, :through => :declarations_users   has_many :my_declarations, :class_name => 'Declarations', :through => :declarations_users

  def fullname     return self.firstname + ' ' + self.lastname   end

end

Is it possible to add a new controller and automatically add the appropriate users in the declarations_payers table? Is was trying to do something like this:   <%= check_box_tag 'declaration[declarations_payer.user]', user.id %>

But it just didn't work...

Look here for my previous question about this application: http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/6fe2827af57a4123

Thanks in advance!