testing modules in isolated

policy_spec.rb

require 'rails_helper'

describe Policy do
  let!(:user) { create :user }
  let(:other_user) { create :user }
  
  # создаем класс
  class PolicyClass
    # подключаем модуль который необходимо протестировать
    include Policy
    
    # добавляем пустой метод
    def user
    end
  end

  describe '#can_destroy?' do
    policy = PolicyClass.new

    it 'return true' do
      allow(policy).to receive(:user) { user }

      expect(policy.can_destroy?(user)).to be true
    end

    it 'return false' do
      allow(policy).to receive(:user) { user }
      
      expect(policy.can_destroy?(other_user)).to be false
    end
  end
end