pseudo code

class Discount
  # name
  # code
  # effective_at
  # ends_at
  # type             # (DollarOff, PercentOff, FreeShipping..)
  # promotion?
  # customer_exclusive?
  # amount
  # minimum_total_requirement

  has_many :assignments, :class_name => "DiscountAssignment"
  has_many :users, :through => :assignments
  has_many :sales_orders, :through => :assignments

  has_many :filters

  named_scope :promotions, :conditions => { :promotion => true }
  named_scope :customer_credits, :conditions => { :promotion => false }
  named_scope :global, :conditions => { :customer_exclusive => false }

  def filter(order)
    order.details.select do |d|
      filters.all? {|f| f.is_applicable? d }
    end
  end

  def applicable?(order)
    active? &&
    order.total >= minimum_total_requirement &&
    # check dates, yadda, yadda
    filter(order).any? # do ANY details pass ALL the filters?
  end
end
  

DiscountAssignment
  promo_id
  user_id
  sales_order_id

class DiscountFilter < ActiveRecord::Base
  # criterion

  # interface definition
  def is_applicable?(order_detail)
    raise "Implement this in your subclass"
  end
end

class ColorDiscountFilter < ActiveRecord::Base
  def is_applicable?(order_detail)
    order_detail.product.color == self.criterion
  end
end