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class User
  has_many :subscriptions
  has_many :subscribables, :through => subscriptions
end

class Subscription
  belongs_to :user
  belongs_to :subscribable, :polymorphic => true
end

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

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