waaah.rb

class Foo < ActiveRecord::Base
  # attrs: bar (integer), baz (integer)

  # want to make sure Foo.baz is > than Foo.bar
  # this would only run validation when baz is numeric (because any int is > nil)
  validates_each :bar do |record, attr, value|
    if record.baz.is_a?(Numeric)
      if value.is_a?(Numeric)
        record.errors.add attr, 'must be less than baz.' unless value < record.baz
      end
    end
  end
  
end