A simple Discount domain model with validations:

  Class Discount
    belongs_to :promo_code
    belongs_to :customer
    belongs_to :order

    validates_presence_of :promo_code, :customer, :order
    validates_associated :promo_code
    validates_uniqueness_of :promo_code_id, :scope => [:customer_id, :order_id]
  end

Limiting down to a single Promo-code per order:

  Class Discount
    # ...
    validates_uniqueness_of :order_id, :scope => :customer_id
  end