为啥“.order（”created_at DESC“）”在Rspec中搞砸了“允许”？

Posted 2023-02-15

【中文标题】为啥“.order（”created_at DESC“）”在Rspec中搞砸了“允许”？

【英文标题】：Why does ".order("created_at DESC")" mess up "allow" in Rspec?为什么“.order（”created_at DESC“）”在Rspec中搞砸了“允许”？

【发布时间】：2021-12-17 09:47:28

【问题描述】：

在 Rspec 中遇到问题。假设我有这个：

class Book; has_many: :pages; end
class Page; belongs_to: :book; end


describe Pages
  let(:book)  create(:book) 
  let(:page)  create(:page, book: book) 
  before do
    allow(page).to receive(:last_page?).and_return(last_page)
    book.pages << page
  end

  context "weird behavior" do
    let(:last_page)  "Orange" 
    
    it do
      # these two pass as expected
      expect(book.pages.first).to eq page # passes, as expected
      expect(book.pages.first.last_page?).to eq "Orange" # passes, as expected

      # this is where weird things happen
      expect(book.pages.order("created_at DESC").first).to eq page # passes, as expected
      expect(book.pages.order("created_at DESC").first.last_page?).to eq "Orange" # will fail and return the actual method call
    end
  end
end

为什么 ".order("created_at DESC")" 会弄乱 "allow" 语句，即使实际对象仍然相等？

【问题讨论】：

如果这是整个规范文件 expect(book.pages.first.last_page?).to eq "Orange" 没有通过，last_page? 仍然会从未被存根的 book.pages.first 调用。

也许澄清你的意图会引导你找到更好的方法。

.last_page? 应该按照 Ruby 约定返回一个布尔值。它返回一个字符串的事实本身就是一个 WTF 时刻。

@SebastianPalma 你是对的，那条线实际上没有通过。但是，当我在控制台中调试时，如果我运行book.pages 然后book.pages.first.last_page?，我会得到“橙色”。如果我运行“book.pages.reload.first.last_page?”，那么它会返回到方法调用。知道为什么吗？

【参考方案1】：

我可以通过这个。

describe Pages
  let(:book)  create(:book) 
  let(:page)  create(:page, book: book) 
  before do
    # here you create the expectation on the page object
    allow(page).to receive(:last_page?).and_return(last_page)
    # and append the page to the pages collection
    # this collection / scope could be considered to be already loaded from the DB if you like
    book.pages << page
  end

  context "weird behavior" do
    let(:last_page)  "Orange" 
    
    it do
      # these two pass as expected
      # and here you are simply extracting the page object back from what is effectively a loaded pages scope.
      expect(book.pages.first).to eq page # passes, as expected
      expect(book.pages.first.last_page?).to eq "Orange" # passes, as expected

      # it's equivalent to
      expect(page).to eq page # passes, as expected
      expect(page.last_page?).to eq "Orange" # passes, as expected

      # I would expect this to fail since the first page would be different to the object you created the expectation on
      expect(book.pages.reload.first).to eq page # passes, as expected
      expect(book.pages.reload.first.last_page?).to eq "Orange" # passes, as expected

      # this is where weird things happen
      # not so weird.
      # book.pages is one scope object
      # book.pages.limit(1) would be another
      # book.pages.order('id') would be another
      # book.pages.order('created_at DESC') would fetch from the database and that object would not have your expectation on it
      # expections can only be made on an object not on every instance of an object with that id.

    
      expect(book.pages.order("created_at DESC").first).to eq page # passes, as expected
      expect(book.pages.order("created_at DESC").first.last_page?).to eq "Orange" # will fail and return the actual method call
    end
  end
end

【讨论】：

感谢您彻底解释此@RobL。我想我现在明白其中的区别了。我看到的另一件我不明白的事情是：当我运行book.pages.first.last_page? 时，它实际上返回了方法调用（不是“橙色”，我在原始帖子中错误地说。）但是，如果我第一次运行book.pages，然后book.pages.first.last_page?，然后它确实返回“橙色”。如果我再做book.pages.reload.first.last_page?，我会再次得到方法调用。为什么会这样？

【参考方案2】：

发生了什么：

book.pages.first.last_page?

first 返回之前块中设置的对象：

book.pages << page

所以当你运行 allow(page) 和 book.pages.first 他们指向同一个对象时，试试

page.object_id == book.pages.first.object_id # true

但是当你打电话时

book.pages.order("created_at DESC").first

Rails 正在从数据库中获取所有相关页面并构建全新的对象，所以

page.object_id == book.pages.order("created_at DESC").first.object_id # false

所以模拟对象page 是与book.pages.order("created_at DESC").first.object_id 不同的对象，后者只是调用在Page 模型上实现的原始方法last_page?。

换句话说：你的 mock 是针对内存中的这个特定对象，调用 order 会为 DB 获取记录并在内存中创建不同的对象，而你没有在这些对象上设置 mock。

有很多方法可以修复它（更好或更糟，取决于您正在设计的系统的其他一些特性），这值得一个完整的单独堆栈溢出问题恕我直言（随意发布！）。但我希望你清楚为什么你的一个期望通过而另一个失败。