validates_uniqueness_of: passing a method to :scope

I was wondering if anybody knows an easy way to pass a method name as
the scope option to a validates_uniqueness_of validation on a AR model?

For example, I have nested models a bit like:

class Survey
  has_many :areas
end

class Area
  belongs_to :survey
  has_many :questions, :order => 'position'
  validates_uniqueness_of :position, :scope => : survey_id
end

class Question
  belongs_to :area
  validates_uniqueness_of :position, :scope => : area_id
end

So to get the questions on a survey, sorted by the 'survey_position'
attribute I have to add a foreign key to the question, so that I can
pass the survey_id to the :scope argument on validates_uniquess_of. So,
I have to add this stuff to Survey and Question classes:

class Survey
  has_many :questions, :order => 'questions.survey_position'
end

class Question
  belongs_to :survey
  validates_uniqueness_of :survey_position, :scope => :survey_id
end

This creates problems for when saves cascade down associated objects.

Any ideas on how to pass a method name as the scope option? Then I could
add (instead of the above):

class Survey
  def questions
    find_all_questions.sort_by(:survey_position)
  end
end

class Question
  validates_uniqueness_of :survey_position, :scope =>
:survey_id_from_area

  def survey_id_from_area
    area.survey.id
  end
end

Any ideas?

~ Mark