假如遇到一段想重構的代碼，但不幸的是：

1. 看不懂在幹嘛
2. 沒有被測試給覆蓋

重構自己不理解的程式碼是有巨大的風險的，而且問題來了，如果不知道它是做什麼的，那我要怎麼幫這段代碼寫測試呢？

有一個有趣的東西叫做：表徵測試 ( Characterization Test )，之前讀文章的時候看到的，它讓解決難題變得直接和簡單，等等會用一些簡單的範例做示範！

假設情境

假設這是一段舊專案的程式碼。

下面的兩個方法 services_with_info & products_with_info 有幾個問題。

一個是他們具有很大的重複性，還有這兩個方法其實沒什麼用，應該可以放到其他地方去，讓 Appointment 專注當一個 Appointment 的 Model，不必把 appointment_services & appointment_products 序列化的過程放到 Model 裏面。

class Appointment < ActiveRecord::Base
  def services_with_info
    services.collect { |item|
      item.serializable_hash.merge(
        "price": number_with_precision(item.price, precision: 2),
        "label": item.service.name,
        "item_id": item.service.id,
        "type": "service"
      )
    }
  end

  def products_with_info
    products.collect { |item|
      item.serializable_hash.merge(
        "price": number_with_precision(item.price, precision: 2),
        "label": item.product.name,
        "item_id": item.product.id,
        "type": "product"
      )
    }
  end
end

重構 services_with_info

這兩個方法中，我們先把目光放在 services_with_info，目標是抽離這個方法，創立一個抽象的概念，然後把主體移過去那邊。

重構時可能可以第一次就想到一個很棒的抽象概念，也可能不行。我們可以先試試看一個抽象概念叫做 Appointment::ServiceCollection 的新類別。

下面是這個類別，目前做的就只是把 services_with_info 的內容複製貼上過來而已。

module Appointment
  class ServiceCollection
    def initialize(services)
      @services = services
    end

    def to_h
      @services.to_h { |item|
        item.serializable_hash.merge(
          "price": number_with_precision(item.price, precision: 2),
          "label": item.service.name,
          "item_id": item.service.id,
          "type": "service"
        )
      }
    end
  end
end

簡單的實體化物件

當開始為這個新類別寫測試時，盡可能投入較少的經歷，先問自己一個問題，這個 ServiceCollection 能夠實體化物件嗎？

RSpec.describe Appointment::ServiceCollection do
  describe '#to_h' do
    it 'works' do
      appointment_service = create(:appointment_service)
      collection = Appointment::ServiceCollection.new([appointment_service])
    end
  end
end

被測試的方法能夠回傳值嗎?

剛剛的測試通過後，下一步將是期待一個看起來有點蠢的東西

RSpec.describe Appointment::ServiceCollection do
  describe '#to_h' do
    it 'works' do
      appointment_service = create(:appointment_service)
      collection = Appointment::ServiceCollection.new([appointment_service])
      expect(collection.to_h).to eq('asdf')
    end
  end
end

我當然知道 collection.to_h 不會等於 'asdf'，但我不知道他等於什麼，所以任何的回傳值都會是錯誤。完全沒必要花心思去猜他會回傳什麼，反正測試結果會告訴我！

F

Failures:

  1) Appointment::ServiceCollection#to_h works
     Failure/Error: expect(collection.to_h).to eq('asdf')
     TypeError:
       wrong element type AppointmentService at 0 (expected array)
     # ./app/models/appointment/service_collection.rb:7:in `to_h'
     # ./app/models/appointment/service_collection.rb:7:in `to_h'
     # ./spec/models/appointment/service_collection_spec.rb:8:in `block (3 levels) in <top (required)>'

Finished in 0.50943 seconds (files took 3.44 seconds to load)
1 example, 1 failure

因為是重構一個不知道會回傳什麼的方法，所以錯誤很正常，我們可以嘗試把程式碼註解掉。

module Appointment
  class ServiceCollection
    def initialize(services)
      @services = services
    end

    def to_h
      @services.to_h { |item|
        #item.serializable_hash.merge(
          #"price": number_with_precision(item.price, precision: 2),
          #"label": item.service.name,
          #"item_id": item.service.id,
          #"type": "service"
        #)
      }
    end
  end
end

結果還是錯誤，那就註解掉更多的程式碼看看

module Appointment
  class ServiceCollection
    def initialize(services)
      @services = services
    end

    def to_h
      #@services.to_h { |item|
        #item.serializable_hash.merge(
          #"price": number_with_precision(item.price, precision: 2),
          #"label": item.service.name,
          #"item_id": item.service.id,
          #"type": "service"
        #)
      #}
    end
  end
end

沒有太意外的還是錯誤，但結果改變了！

F

Failures:

  1) Appointment::ServiceCollection#to_h works
     Failure/Error: expect(collection.to_h).to eq('asdf')

       expected: "asdf"
            got: nil

       (compared using ==)
     # ./spec/models/appointment/service_collection_spec.rb:8:in `block (3 levels) in <top (required)>'

Finished in 0.49608 seconds (files took 3.41 seconds to load)
1 example, 1 failure

很明顯，問題出在 #@appointment_services.to_h { |item| 這行。問題大概是出在 to_h 身上，那我試著用 map 代替呢？

module Appointment
  class ServiceCollection
    def initialize(services)
      @services = services
    end

    def to_h
      @services.map { |item|
        #item.serializable_hash.merge(
          #"price": number_with_precision(item.price, precision: 2),
          #"label": item.service.name,
          #"item_id": item.service.id,
          #"type": "service"
        #)
      }
    end
  end
end

成功了，這樣沒有得到像之前 wrong element type 的錯誤，而是不一樣的，更符合我們想看到的東西。

F

Failures:

  1) Appointment::ServiceCollection#to_h works
     Failure/Error: expect(collection.to_h).to eq('asdf')

       expected: "asdf"
            got: [nil]

       (compared using ==)

       Diff:
       @@ -1,2 +1,2 @@
       -"asdf"
       +[nil]

     # ./spec/models/appointment/service_collection_spec.rb:8:in `block (3 levels) in <top (required)>'

Finished in 0.48778 seconds (files took 3.5 seconds to load)
1 example, 1 failure

現在讓我把之前的註解都全部拔掉看看會怎麼樣：

module Appointment
  class ServiceCollection
    def initialize(services)
      @services = services
    end

    def to_h
      @services.map { |item|
        item.serializable_hash.merge(
          "price": number_with_precision(item.price, precision: 2),
          "label": item.service.name,
          "item_id": item.service.id,
          "type": "service"
        )
      }
    end
  end
end

出現另一個不同的錯誤，說找不到 number_with_precision 這個方法

F

Failures:

  1) Appointment::ServiceCollection#to_h works
     Failure/Error: expect(collection.to_h).to eq('asdf')
     NoMethodError:
       undefined method `number_with_precision' for #<Appointment::ServiceCollection:0x007ff60274e640>
     # ./app/models/appointment/service_collection.rb:9:in `block in to_h'
     # ./app/models/appointment/service_collection.rb:7:in `map'
     # ./app/models/appointment/service_collection.rb:7:in `to_h'
     # ./spec/models/appointment/service_collection_spec.rb:8:in `block (3 levels) in <top (required)>'

Finished in 0.52793 seconds (files took 3.32 seconds to load)
1 example, 1 failure

剛好我以透過 Rails API 知道這個方法是放在哪裡，如果要在獨立的類別中使用，需要 include ActionView::Helpers::NumberHelper 進來用。

module Appointment
  class ServiceCollection
  include ActionView::Helpers::NumberHelper

    def initialize(services)
      @services = services
    end

    def to_h
      @services.map { |item|
        item.serializable_hash.merge(
          "price": number_with_precision(item.price, precision: 2),
          "label": item.service.name,
          "item_id": item.service.id,
          "type": "service"
        )
      }
    end
  end
end

回傳的 value

處理完這個問題後，終於可以看到回傳的結果是什麼：

F

Failures:

  1) Appointment::ServiceCollection#to_h works
     Failure/Error: expect(collection.to_h).to eq('asdf')

       expected: "asdf"
            got: [{"id"=>1, "appointment_id"=>1, "service_id"=>1, "created_at"=>Sun, 24 Feb 2019 16:45:16 EST -05:00, "updated_at"=>Sun, 24 Feb 2019 16:45:16 EST -05:00, "length"=>nil, "stylist_id"=>2, "price"=>"0.00", "label"=>"ve0xttqqfl", "item_id"=>1, "type"=>"service"}]

       (compared using ==)

       Diff:
       @@ -1,2 +1,12 @@
       -"asdf"
       +[{"id"=>1,
       +  "appointment_id"=>1,
       +  "service_id"=>1,
       +  "created_at"=>Sun, 24 Feb 2019 16:45:16 EST -05:00,
       +  "updated_at"=>Sun, 24 Feb 2019 16:45:16 EST -05:00,
       +  "length"=>nil,
       +  "stylist_id"=>2,
       +  "price"=>"0.00",
       +  "label"=>"ve0xttqqfl",
       +  "item_id"=>1,
       +  "type"=>"service"}]

     # ./spec/models/appointment/service_collection_spec.rb:8:in `block (3 levels) in <top (required)>'

Finished in 0.72959 seconds (files took 3.24 seconds to load)
1 example, 1 failure

看到回傳的結果真的很有趣，因為這是在重構一個舊專案，看到回傳值的時候就覺得好像要解開秘密了！

使用更直接的 Expectation

我們要用更現實的數值來進行測試。我知道測試會失敗，所以數值是隨意捏造的。

RSpec.describe Appointment::ServiceCollection do
  describe '#to_h' do
    it 'works' do
      appointment_service = create(:appointment_service)
      collection = Appointment::ServiceCollection.new([appointment_service])

      item = collection.to_h.first
      expect(item['price']).to eq('30.00')
      expect(item['label']).to eq('Robert Chang')
      expect(item['item_id']).to eq(appointment_service.item.id)
      expect(item['type']).to eq('service')
    end
  end
end

這是為了讓我的測試設置更符合我們期待的值！

RSpec.describe Appointment::ServiceCollection do
  describe '#to_h' do
    it 'works' do
      appointment_service = create(
        :appointment_service,
        price: 30,
        service: create(:service, name: 'Robert Chang'))

      collection = Appointment::ServiceCollection.new([appointment_service])

      item = collection.to_h.first
      expect(item['price']).to eq('30.00')
      expect(item['label']).to eq('Robert Chang')
      expect(item['item_id']).to eq(appointment_service.item.id)
      expect(item['type']).to eq('service')
    end
  end
end

測試通過後,我要做什麼?

現在測試通過了，到目前為止，我們盡可能減少接觸原本程式碼的機會，因為我想最大限度的保留原本功能。現在我有了測試，我的測試可以保有原本的功能，而且我還可以自由地重構這段程式碼！

module Appointment
  class ServiceCollection
    include ActionView::Helpers::NumberHelper
    ITEM_TYPE = 'service'

    def initialize(services)
      @services = services
    end

    def to_h
      @services.map do |service|
        service.serializable_hash.merge(extra_attributes(service))
      end
    end

    private

    def extra_attributes(service)
      {
        'price'   => number_with_precision(service.price, precision: 2),
        'label'   => service.service.name,
        'item_id' => service.service.id,
        'type'    => ITEM_TYPE
      }
    end
  end
end

順便來整理一下測試的代碼：

RSpec.describe Appointment::ServiceCollection do
  describe '#to_h' do
    let(:appointment_service) do
      create(
        :appointment_service,
        price: 30,
        service: create(:service, name: 'Robert Chang')
      )
    end

    let(:item) { Appointment::ServiceCollection.new([appointment_service]).to_h.first }

    it 'adds the right attributes' do
      expect(item['price']).to eq('30.00')
      expect(item['label']).to eq('Robert Chang')
      expect(item['item_id']).to eq(appointment_service.service.id)
      expect(item['type']).to eq('service')
    end
  end
end

最後，我們就可以用我們新做的類別來取代原先的程式碼：

# 原本的

def services_with_info
    services.collect { |item|
      item.serializable_hash.merge(
        "price": number_with_precision(item.price, precision: 2),
        "label": item.service.name,
        "item_id": item.service.id,
        "type": "service"
      )
    }
end

# 現在的

def services_with_info
  Appointment::ServiceCollection.new(services).to_h
end

結語

這在重構的路上可以說是杯水車薪，但現在我們已經減少了很多的程式碼，因為其實 Rails 的 MVC 也沒有那麼夠用，並不是真的讓 Model 肥就是好，像這種取得資訊的方法，我們說不定可以把它全部拉出來整理，放到 Presenter 的層級。

但至少我們今天也知道了，如果一段程式碼沒有測試，而且還不知道它會回傳什麼的時候，嘗試看看這個表徵測試的方式，一步一步的找到能夠整理的邏輯，並且同時補上測試！