Dynamic model validation

Hi, I am searching for a way to dynamic validate my models.

The solution that I got working is:

def after_initialize
  singleton = class << self; self; end

  validations = eval(calendar.cofig)

  validations.each do |val|
    singleton.class_eval(val)
  end
end

On my actual app, I have 2 models

class Calendar < ActiveRecord::Base
  has_many :events
end

class Event < ActiveRecord::Base
  belongs_to :calendar

  def after_initialize
    singleton = class << self; self; end

    validations = eval(calendar.config)

    validations.each do |val|
       singleton.class_eval(val)
    end
end

end

As you can see, the validation code that should be added to the Event
class lies on the Calendar field "config".

Works fine for a existing Event, but doesn't for a new record. That's
because, at the time that after_initialize is called, the association
doesn't exists yet.

I can't find a way to do that besides putting the config values on
Event itself.

Any advices?

Tks!