Mock Spec

spec.rb

class CategoryCacheClearer < Struct.new(:article)
  def call
    article.category_list = ''
    article.save
  end
end

describe CategoryCacheClearer do
  describe '#call' do
    let(:article) { mock(:save, :category_list=) }
    let(:category_cache_clearer) { CategoryCacheClearer.new(article) }

    it 'clears the category list' do
      article.should_receive(:category_list=)
      category_cache_clearer.call
    end
    it 'calls save on the article' do
      article.should_receive(:save)
      category_cache_clearer.call
    end
  end
end

I tend to use bourne to spy on behavior like this so it'd be more like this:

it 'clears the category list' do
  category_cache_clearer.call

  expect(article).to have_received(:category_list=).with('')
end

It just reverses the steps so that you're keeping the standard four phase test (setup, execute, assert, teardown). You also want to ensure that you're testing the full interface on outgoing command messages, so ensuring that you're doing with('') is somewhat important.