```ruby
require "rails_helper"

RSpec.describe User, :type => :model do
  it "orders by last name" do
    lindeman = User.create!(first_name: "Andy", last_name: "Lindeman")
    chelimsky = User.create!(first_name: "David", last_name: "Chelimsky")

    expect(User.ordered_by_last_name).to eq([chelimsky, lindeman])
  end
end```

Add tests for article model
user
```ruby
require "rails_helper"

describe Article do
  let(:user) { User.new(username: "sampleuser", permission_level: "author", password: "password") }
  let(:article) { Article.new(orig_author: :user, title: "When Awesome Groups Make Awesome Apps") }

  describe '#orig_author' do
    it 'must have an original author' do
      expect(article.orig_author).not_to be nil
    end

    it 'has a user as its original author' do
      expect(article.orig_author).to_be_instance_of(User)
    end

    it 'returns the user who authored the article' do
      expect(article.orig_author).to eq(:user)
    end
  end

  describe '#title' do
    it 'must have a title' do
      expect(article.title).not_to be nil
    end

    it 'returns the title of the article' do
      expect(article.title).to eq("When Awesome Groups Make Awesome Apps")
    end
  end
end```
assistant