くらそうねに実在した不安定なRSpecのテストとその倒し方

class Appointment < ApplicationRecord
  validate :scheduled_date_must_be_business_day, if: :scheduled_date

  class << self
    def holiday?(date)
      # 祝日かどうかの判定にはHolidayJpというライブラリを使っています。
      # https://github.com/holiday-jp/holiday_jp-ruby
      date.sunday? || HolidayJp.holiday?(date)
    end
  end

  private
    def scheduled_date_must_be_business_day
      errors.add(:scheduled_date, :not_business_day) if Appointment.holiday?(scheduled_date)
    end
end

RSpec.describe Appointment do
  describe '.holiday?' do
    let(:date) { Date.current }

    subject { described_class.holiday?(date) }

    it { is_expected.to be false }
  end
end

RSpec.describe Appointment do
  describe '.holiday?' do
    let(:non_holiday_monday) { Date.current.beginning_of_week }
    let(:non_holiday_sunday) { Date.current.end_of_week }

    # 来年の今日までに月曜日の祝日があることを前提にしている。
    let(:holiday_monday) { HolidayJp.between(Date.current, Date.current.next_year).filter(&:monday?).first.date }

    before do
      # テストをさらに安定化するためにHolidayJp.holiday?の判定をスタブする。
      allow(HolidayJp).to receive(:holiday?).with(non_holiday_monday).and_return(false)
      allow(HolidayJp).to receive(:holiday?).with(non_holiday_sunday).and_return(false)
    end

    context '平日を渡したとき' do
      subject { described_class.holiday?(non_holiday_monday) }
      it { is_expected.to be false }
    end

    context '日曜日を渡したとき' do
      subject { described_class.holiday?(non_holiday_sunday) }
      it { is_expected.to be true }
    end

    context '祝日を渡したとき' do
      subject { described_class.holiday?(holiday_monday) }
      it { is_expected.to be true }
    end
  end
end

class Appointment < ApplicationRecord
  # created_atの降順に並べ替えるスコープ。
  scope :order_by_created_at, -> { order(created_at: :desc) }
end

RSpec.describe Appointment do
  describe '.order_by_created_at' do
    let!(:first_appointment) { Appointment.create }
    let!(:second_appointment) { Appointment.create }
    let!(:third_appointment) { Appointment.create }

    subject { Appointment.order_by_created_at }

    it { is_expected.to eq [third_appointment, second_appointment, first_appointment] }
  end
end

RSpec.describe Appointment do
  describe '.order_by_created_at' do
    let!(:first_appointment) { Appointment.create(created_at: Time.current) }
    let!(:second_appointment) { Appointment.create(created_at: Time.current - 1.hour) }
    let!(:third_appointment) { Appointment.create(created_at: Time.current + 1.hour) }

    subject { Appointment.order_by_created_at }

    it { is_expected.to eq [third_appointment, first_appointment, second_appointment] }
  end
end