review_spec.rb

RSpec.describe Review do
  let(:book) { Book.create! }
  subject { Review.new(book: book) }

  it "increments the book's reviews_count when saving" do
    expect {
      subject.save!
    }.to change(book, :reviews_count).by(1)
  end

  it "decrements the book's reviews_count when destroying" do
    subject.save!

    expect {
      subject.destroy!
    }.to change(book, :reviews_count).by(-1)
  end

  it "moves the entry when changing book" do
    book2 = Book.create!

    subject.save!

    expect {
      subject.update! book: book2
    }.to change(book, :reviews_count).by(-1) && change(book2, :reviews_count).by(1)
  end
end