On page 292, we create a SupportRequest model that belongs_to Order

class SupportRequest < ApplicationRecord
  belongs_to :order, optional: true
end

We also need to add the has_many end of the relationship to orders or else you will get a foreign key error when you attempt to destroy an Order that has a SupportRequest associated with that order id.

class Order < ApplicationRecord
  has_many :line_items, dependent: :destroy
  # need this relationship defined or it will break orders#destroy
  # since the relationship is optional on the SupportRequest end, it should nullify
  has_many :support_requests, dependent: :nullify
  # rest of class
end