Rails Ruby on Rails

I tried to implement a dirty filter in a model in rails3-rc2, ruby 1.9.2 to call a procedure if certain attributes were changed.

Even though I could not find documentation in edgeapi (other then ActiveModel::dirty) that was close to ActiveRecord:dirty rails2 documentation - I figured that the "changed?" method was there.

my after_save filter

def updateMax
  computeMax(self.assessment.id) if is_dirty?
end

def is_dirty?
  dirty =  (self.score_method_changed? or 
    self.weight_changed? or 
    self.critical_changed? or 
    self.minimum_value.changed?)
 end

The weight attribute is a decimal number (psql). If I update another field besides score_method(string), weight(decimal), critical(boolean), or minimum_value(integer) I get an error:

undefined method `changed?' for 2:Fixnum
Rails.root: /Users/salex/work/aidtdb3r3

Application Trace | Framework Trace | Full Trace
app/models/question.rb:37:in `is_dirty?'
app/models/question.rb:10:in `updateMax'
app/controllers/questions_controller.rb:65:in `block in update'
app/controllers/questions_controller.rb:64:in `update'

I also get the error if I update minimum_value without updating weight. But updating score_method or critical without updating weight works.

I got around it by getting the self.changed array and iterating thru the array to see if one of my fields was in the array.