ruby-on-rails - RSpec "count"change.by？

Question

所以我在看:https://rubyplus.com/articles/1491-Basic-TDD-in-Rails-Writing-Validation-Tests-for-the-Model

只是看到测试技术，我看到了这个:

require 'rails_helper'

describe Article, type: :model do
  it 'is valid if title and description fields have value' do
    expect do
      article = Article.new(title: 'test', description: 'test')
      article.save
    end.to change{Article.count}.by(1)
  end
end

特别是最后一行: end.to change{Article.count}.by(1) .来自阅读 https://relishapp.com/rspec/rspec-expectations/v/3-7/docs/built-in-matchers/change-matcher

它具体说:

The change matcher is used to specify that a block of code changes some mutable state. You can specify what will change using either of two forms:

这是有道理的。但是正在测试 Article.count在实际上没有“做”任何事情的代码块中( article.save 实际改变了 Article.count 那么这究竟是如何工作的？测试是否在运行之前查看代码块中的内容并“预运行”它...比较 .by(1) 之后？

谢谢

Answer 1

有两个代码块正在执行。传递给 expect 的代码块，以及传递给 change 的代码块.这是真正发生的事情，在伪代码中。

difference = 1
initial_count = Article.count

article = Article.new(title: 'test', description: 'test')
article.save

final_count = Article.count

expect(final_count - initial_count).to eq(difference)

我会将您的测试重构为更容易遵循，如下所示:

require 'rails_helper'

describe Article, type: :model do
  let(:create_article) { Article.create(title: 'test', description: 'test') }

  it 'is valid if title and description fields have value' do
    expect { create_article }.to change { Article.count }.by(1)
  end
end