Add spec that was left out of previous commit
user
```ruby
require 'rails_helper'

describe Alert do
  let(:article) { create(:article) }
  let(:course) { create(:course) }
  let(:revision) { create(:revision) }
  let(:user) { create(:user) }

  describe 'abstract parent class' do
    it 'raises errors for required template methods' do
      alert = Alert.new
      expect { alert.main_subject }.to raise_error(NotImplementedError)
      expect { alert.url }.to raise_error(NotImplementedError)
    end
  end

  describe 'types' do
    it 'all implement #main_subject' do
      Alert::ALERT_TYPES.each do |type|
        Alert.create(type: type,
                     article_id: article.id,
                     course_id: course.id,
                     revision_id: revision.id,
                     user_id: user.id)
        expect(Alert.last.main_subject).to be_a(String)
      end
    end

    it 'all implement #url' do
      Alert::ALERT_TYPES.each do |type|
        Alert.create(type: type,
                     article_id: article.id,
                     course_id: course.id,
                     revision_id: revision.id,
                     user_id: user.id)
        expect(Alert.last.url).to be_a(String)
      end
    end
  end
end
```
assistant