datevalidations.rb

class Invoice < ActiveRecord::Base
  validate :expiration_date_cannot_be_in_the_past,
    :discount_cannot_be_greater_than_total_value
 
  def expiration_date_cannot_be_in_the_past
    if !expiration_date.blank? and expiration_date < Date.today
      errors.add(:expiration_date, "can't be in the past")
    end
  end
 
  def discount_cannot_be_greater_than_total_value
    if discount > total_value
      errors.add(:discount, "can't be greater than total value")
    end
  end
end