I'm trying to get two parent documents to reference the same child like so:

class PaymentNotification
  has_many :items, as: :itemable
end

class Product
  has_many :items, as: :itemable
end

class Item
  belongs_to :itemable, polymorphic: true
end

My issue is when I save an Item to a PaymentNotification, it gets removed from the Product that it had previously belonged to (and vice versa).