assert_difference expecting incorrect count, fails as a result

Question

A Book has_many Reviews and a Review belongs_to Book. I am using rails 4.0 and ruby 2.0.0p247

I am running the test as follows:

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

As you can see, I have printed the count, before and after deleting the review associated with a book, and it decreases by 1. However, the assertions expects and receives incorrect counts, showing this as the error:

 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

I have dropped the db and started with a blank one. I have two questions:

1. I have only created one review associated with one book. Why is the Review.count printing 3 and 2 as against 1 and 0?

2. Why does the assertion expect 4, but receive 2?

Thanks.

Answer 1

You have 3 records. If you write assert_difference 'Review.count', 1... it expects 4 at the end. But if you write assert_difference 'Review.count', -1... it expects 2, thus 1 less.

Answer 2

This line tells that you are expecting the difference of Plus one(1 or +1).

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

But actually you want the difference in Negative one(-1), so you should write

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

For more guide visit assert_difference