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class Subscription
  has_many :subscription_items
  has_many :subscribables, :through => subscription_items
end

class SubscriptionItem
  belongs_to :subscription
  belongs_to :subscribable, :polymorphic => true
end

class XProduct
  has_many :subscription_items, :as => :subscribable
end

class YProduct
  has_many :subscription_items, :as => :subscribable
end