Have a product that belongs to a category. Want to create a promotion for a short period of time (lets say a week or two), but their can be only one promotion per category during that time.

How can I create a custom validation for this?

product class
  belongs_to :categories

  name:string
  desc:text  
  reg_price:decimal  
  category_id:integer  
  promo_active:boolean  
  promo_price:decimal  
  promo_start:datetime  
  promo_end:datetime
end

category class
  has_many :products

  name:string
end