app/models/concerns/destroy_if_zero_quantity.rb


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module DestroyIfZeroQuantity
  extend ActiveSupport::Concern

  included do
    after_save :destroy_if_zero_quantity
  end

  private
    def destroy_if_zero_quantity
      if self.quantity == 0
        destroy
      elsif self.quantity < 0
        # TODO: Can improve this (at the least, with reporting, later).
        raise ItemQuantityError
      end
    end
end