create_book.rb

class CreateBook
  def initialize(title, author_guid)
    @title, @author_guid = title, author_guid
  end
  
  def perform
    unless author
      log "Could not find author"
    else
      book.save!
    end
  end
  
  def book
    @book ||= Book.new title: @title, author: author
  end
  
  def author
    return @author if defined?(@author)
    @author = Author.find_by_guid(@author_guid) || ExternalStorage.import_author(@author_guid)
  end
end

create_book_spec.rb

describe CreateBook do
  let(:op){ described_class.new "Lord of the Rings", "001" }
  
  describe "#perform" do
    let(:author){ create :author }
    
    before { allow(op).to receive(:author){ author } }
    
    it "saves the book" do
      expect{ op.perform }.to change{ op.book.new_record? }.to(true)
    end
    
    context "when author not found" do
      let(:author){ nil }
      
      it 'does not create anything' do
        expect{ op.perform }.to_not change{ Book.count }
      end
    end
  end
  
  describe '#book' do
    let(:author){ create :author }
    
    before { allow(op).to receive(:author){ author } }
    
    it 'has proper attributes' do
      expect(op.book).to have_attributes(title: "Lord of the Rings", author: author)
    end
  end
  
  describe '#author' do
    subject { op.author }
    
    context "when author with given guid exists" do
      let!(:author){ create :author, guid: "001" }
      
      it 'returns that author' do
        should eq author
      end
    end
    
    context "when author does not exist" do
      let(:author){ double :author }
      
      it "imports author from  ExternalAtorage" do
        expect(ExternalStorage).to receive(:import_author).with("001"){ author }
        should eq author
      end
    end
  end
end