A classic finder returning nil

publishes_posts.rb

 Post = Class.new          # Just a placeholder to make the tests run

class PublishesPosts < Struct.new(:post_id, :ui)

  def run
    post = Post.find_by_id(post_id)
    if post
      post.publish
      ui.post_published(post)
    else
      ui.no_such_post(post_id)
    end
  end

end

publishes_posts_spec.rb

describe PublishesPosts do
  let(:post_id) { 137 }
  let(:ui) { stub(:controller) }
  subject { PublishesPosts.new(post_id, ui) }

  after do
    subject.run
  end

  context 'when the post exists' do
    let(:post) { stub(:post) }

    it 'publishes the post' do
      Post.should_receive(:find).with(post_id).and_return(post)
      post.should_receive(:publish)
      ui.should_receive(:post_published).with(post)
    end
  end

  context 'when the post cannot be found' do
    it 'reports the error to the UI' do
      Post.should_receive(:find).with(post_id).and_return(nil)
      ui.should_receive(:no_such_post).with(post_id)
    end
  end

end