Rails - Jun 2006 - callback object problem

I''m trying to create a separate class to do validation so that about 10
models can use the same date validation.  I have tried to use the 
example from the agile book but it''s not working.  Here''s what
I have:

app/models/date_callbacks.rb
class DateCallbacks

	def before_validation(model)
		if model.start_date > Date.today
			model.errors.add(:start_date, "must be before today")
		end
		if !model.end_date.nil?
			if model.end_date > Date.today
				model.errors.add(:end_date, "must be before today")
			end
	   end
		if !model.end_date.nil?
			if model.end_date < model.start_date
				model.errors.add(:end_date, "must be later than start date")
			end
		end
	end
end

app/models/address_history.rb
class PostgradTraining < ActiveRecord::Base
	before_validation DateCallbacks.new
       ....
end

the error i get is:
Callbacks must be a symbol denoting the method to call, a string to be 
evaluated, a block to be invoked, or an object responding to the 
callback method.


thanks for any help,

Nate

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I''m sorry to bump this; but, i am still having trouble with it.

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