Acts_as_audited, many to many relation

Hi all,
I’m having and issue with a many to many relationship. This is my
scenario:
I have a user model, a role model and a role_user model.
Looks something like this:

class User < ActiveRecord::Base
acts_as_audited :protect => false
attr_accessor :password
has_many :role_users
has_many :roles, :through => :role_users

end

class Role < ActiveRecord::Base
acts_as_audited
has_many :role_users
has_many :users, :through => :role_users

end

class RoleUser < ActiveRecord::Base
acts_as_audited
belongs_to :role
belongs_to :user

end

In the user edit form I show one checkbox for each role in order to
grant or
remove roles to a user. I would like this changes to be auditable.
Right now when I check a checkbox and save a new record appears in the
audits table, since a record had been inserted in role_users, but
nothing
happend when I uncheck a role and save (a record is being deleted from
role_users).
It will be great if this could be part of a User change instead of a
RoleUser, since it will be a new version of the user, but this is not
really
a must. More like a nice to have.

Thanks to all in advance and sorry to bother with this newbie question.