我在测试依赖于Singleton类的功能时遇到问题。该类（ERPDao）是具有不同方法的套件，可帮助应用程序使用法拉第gem与外部ERPvíaREST服务进行连接。 URLMaker是用于构建请求字符串的帮助程序类。当我尝试运行依赖于此方法之一的功能规格时，我在rspec中收到以下消息：

 Failure/Error: result = ERPDao.instance.get_credit_info(erp_id)

 NoMethodError:
   undefined method `instance' for ERPDao:Class
   Did you mean?  instance_of?
   Did you mean?  instance_of?

我的班级ERPDao

class ERPDao
def initialize
    @end_points = EndPoint.first
    @connection = Faraday.new(:url => @end_points.url_base,request: {
      open_timeout: 10,# opening a connection
      timeout: 10         # waiting for response
    })
end
@@instance = ERPDao.new

def self.instance
    return @@instance
end

def get_credit_info(erp_id)
    begin
        return @connection.get URLMaker.instance.get_uri('credit_info',erp_id)
    rescue Faraday::Error::ConnectionFailed => e
        puts "Connection failed: #{e}"
        return 0,false,0
    end
end

    ...
end

我的rails_helper.rb

require 'spec_helper'
ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment',__FILE__)
abort("The Rails environment is running in production mode!") if Rails.env.production?
require 'rspec/rails'
require 'support/factory_bot'
require 'support/wait_for_ajax'
activeRecord::Migration.maintain_test_schema!

RSpec.configure do |config|
  config.fixture_path = "#{::Rails.root}/spec/fixtures"
  config.use_transactional_fixtures = false

  config.before :suite do
    Databasecleaner.strategy = :truncation
  end

  config.before :each do
    Databasecleaner.clean
  end
  config.infer_spec_type_from_file_location!
  config.filter_rails_from_backtrace!
  config.include Warden::Test::Helpers

  config.include Devise::TestHelpers,type: :controller

  Shoulda::Matchers.configure do |config|
    config.integrate do |with|
      with.test_framework :rspec
      with.library :rails
    end
  end
  Capybara.javascript_driver = :webkit

  Capybara::Webkit.configure do |config|
    config.debug = false
    config.allow_unknown_urls
    config.skip_image_loading
    config.timeout = 15
    config.raise_javascript_errors = false
  end
end

我的rails版本是4.2.6，ruby 2.3.1，factory_bot 4.8.2和rspec-rails 3.7。 有人知道这个错误吗？ 谢谢！