RSpec的測試依賴於返回的隨機數

Question

一個模塊的方法的請求我在Rails API下列模塊：

module SpeedCalculator 

    def self.adjustment 
    Random.rand(0..0.3) 
    end 

    def self.calculate_adjustment(track, car_speed) 
    case track.surface_type 
     when "asphalt" 
     car_speed - (car_speed * self.adjustment).ceil 
     when "gravel" 
     car_speed - (car_speed * self.adjustment).ceil 
     when "snow" 
     car_speed - (car_speed * self.adjustment).ceil 
     else 
     car_speed 
    end 
    end 
end 

我可以成功地測試該調整方法是這樣的：

require 'rails_helper' 

RSpec.describe SpeedCalculator do 
    include SpeedCalculator 

    it "uses an adjustment value between 0 and 0.3" do 
    expect(SpeedCalculator.adjustment).to be >= 0 
    expect(SpeedCalculator.adjustment).to be <= 0.3 
    end 
end 

有可能作出這樣的API請求：

本地主機：3000/API/V1 /汽車/保時捷911軌=摩納哥

您要求系統計算給定軌道上給定汽車的速度。

所以我需要編寫一個請求規範，對於給定的汽車和軌道，返回正確的值。但是如果calculate_adjustment總是應用一個隨機數，我該怎麼做呢？

我相信，我需要創建一個self.adjustment模擬/存根，因此測試將是這樣的：

it "calculates max_speed_on_track when a valid track name is provided" do 
     Car.create!(name:'Subaru Impreza', max_speed:'120', speed_unit:'km/h') 
     Track.create(name:'Monaco') 

     # here create a dummy version of adjustment 
     # that always returns a fixed value, rather than a random value 
     # and somehow use that dummy for the request? 
     # Since x below needs to be a known value for the test to work. 

     get api_v1_car_path(id: 'porsche-911', track: 'monaco')  
     expect(response).to have_http_status(200) 
     expect(response.body).to include_json(car: {max_speed_on_track: x})  
    end 

Answer 1

我該怎麼做，當calculate_adjustment總是應用隨機數？

我相信，我需要創建一個模擬/存根self.adjustment

究竟！你想要測試的最後一件事是隨機行爲（和隨機失敗）。你想要可重現的結果。所以是的，嘲笑你的RNG。

的簡單事情會是這樣：

expect(Random).to receive(:rand).with(0..0.3).and_return(0.1234) 
# or better 
expect(SpeedCalculator).to receive(:adjustment).and_return(0.1234) 

# then proceed with your test 
get api_v1_car_path(id: 'porsche-911', track: 'monaco') 
... 

進一步改善這裏是不使用控制器的規格來測試業務邏輯。將您的業務邏輯封裝在一個對象中並測試該對象（您可以在其中最大限度地使用依賴注入技術）。而你的控制器會變成這樣的：

class CarsController 
    def show 
    calculator = MaxSpeedCalculator.new(params[:id], params[:track]) 
    render json: calculator.call 
    end 
end