RSpec的测试依赖于返回的随机数

Question

一个模块的方法的请求我在Rails API下列模块：

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 

我可以成功地测试该调整方法是这样的：

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 

有可能作出这样的API请求：

本地主机：3000/API/V1 /汽车/保时捷911轨=摩纳哥

您要求系统计算给定轨道上给定汽车的速度。

所以我需要编写一个请求规范，对于给定的汽车和轨道，返回正确的值。但是如果calculate_adjustment总是应用一个随机数，我该怎么做呢？

我相信，我需要创建一个self.adjustment模拟/存根，因此测试将是这样的：

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

我该怎么做，当calculate_adjustment总是应用随机数？

我相信，我需要创建一个模拟/存根self.adjustment

究竟！你想要测试的最后一件事是随机行为（和随机失败）。你想要可重现的结果。所以是的，嘲笑你的RNG。

的简单事情会是这样：

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') 
... 

进一步改善这里是不使用控制器的规格来测试业务逻辑。将您的业务逻辑封装在一个对象中并测试该对象（您可以在其中最大限度地使用依赖注入技术）。而你的控制器会变成这样的：

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