Assume you have a tree, where you want to have set attributes only, if a 
node is a root. Otherwise the model should not accept entries for a 
specific attribute.

I.e. if you have a tree of categories and want to have a locale attribute 
only on roots you could do s'thing like this:

class Category < ActiveRecord::Base
  validates :locale, :presence => true, :if => :root?
  validates :locale, :absence => true, :unless => :root? 
end

Actually I'm doing this a lot. I'll use s'thing else, if someone has a 
better pattern.

Cheers,

Roberto

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