belongs_to_message.rb

require "bundler/setup"
require 'active_record'
require 'sqlite3'
require 'minitest/autorun'

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Schema.define do
  create_table :gift_cards, force: true do |t|
    t.references :checkout
  end
  create_table :checkouts, force: true do |t|
  end
end

class Checkout < ActiveRecord::Base
  has_many :gift_cards
end

class GiftCard < ActiveRecord::Base
  belongs_to :checkout, required: true
end

checkout = Checkout.create!
g = GiftCard.new
g.valid?
puts g.errors.messages.inspect
# 4.2: {:checkout=>["can't be blank"]}
# 5.0: {:checkout=>["must exist"]}