Forum: Ruby on Rails Exended modules and reject_if in nested_attributes

I have some common class methods in a module  'active_record_additions'
that I load into a class by Activity by using extend 
ActiveRecordAdditions
The Activity class uses nested_attributes_for with a reject_if method 
saved
in the module  'active_record_additions'
The problem is that the class methods in 'active_record_additions' works
when I use them as ordinary class methods, but not as a a method in
reject_if.
I got the message NameError (undefined method
`exists_nested_targeting_group?' for class `Activity'):

Maybe that method cannot be a class method ?
Or can accept_nested use methods in an extend module ?
Or anyone thats can give a hint abot whay it does not work ?

Some code snippets
The class definition
# encoding: utf-8
require 'active_record_additions'
######################################
class Activity < ActiveRecord::Base
  include ActiveModel::Validations
  extend ActiveRecordAdditions
The accept nested call
accepts_nested_attributes_for :targeting_group,  :reject_if =>
:exists_nested_targeting_group?, :allow_destroy => true
The start of the exists_nested_targeting_group? module
   def exists_nested_targeting_group?(attributes={})
     exists=false