ruby-on-rails - assert_difference 期望计数不正确，结果失败

Question

A Book has_many Reviews 和 Review belongs_to Book。我正在使用 rails 4.0 和 ruby​​ 2.0.0p247

我正在运行测试如下:

test "DELETE destroy should decrement review count by 1" do

    book = Book.create({
                         :isbn=>"1234567890",
                         :title=>"Dummies Guide to Rails",
                         :author=>"KP",
                         :description=>"lorem ipsum"
    })

    # assert_not_nil Book.find_by_isbn('1234567890')

    review = Review.create!({
                              :book_id => book.id,
                              :name => "KP",
                              :email => "KP@KP.com",
                              :description => "Good book.",
                              # :book => book
    })

    assert_difference 'Review.count', 1 , "a review should be deleted" do
      puts Review.count # => 3
      delete :destroy, :book_id => book.id, :id => review.id
      assert_response :redirect
      puts Review.count # => 2
    end
    # puts Review.count
    assert_nil Review.find_by_id(review.id)
  end

如您所见，我在删除与一本书相关的评论之前和之后打印了计数，它减少了 1。但是，断言期望并接收到不正确的计数，将其显示为错误:

 1) Failure:
ReviewsControllerTest#test_DELETE_destroy_should_decrement_review_count_by_1 [test/controllers/reviews_controller_test.rb:56]:
a review should be deleted.
"Review.count" didn't change by 1.
Expected: 4
  Actual: 2

我已经删除了数据库并从空白开始。我有两个问题:

1. 我只创建了与一本书相关的评论。为什么 Review.count 打印的是 3 和 2 而不是 1 和 0？

2. 为什么断言期望 4，却收到 2？

谢谢。

Answer 1

此行表示您期望加一(1 或 +1)的差值。

assert_difference 'Review.count', 1 , "a review should be deleted"

但实际上你想要负一(-1)的差异，所以你应该写

assert_difference 'Review.count', -1 , "a review should be deleted"

更多指南请访问assert_difference