01.rb

# Example One: spec/unit/lib/space/flight/ship_gateway_spec.rb

RSpec.describe Space::Flight::ShipGateway do
  let(:ship) { FactoryBot.create(:ship) }
  let(:gateway) { described_class.new(ship_repository: Ship) }

  context 'when finding a ship by id' do
    subject { gateway.find_by_id(ship.id) }

    it 'returns ship with correct id' do
      expect(subject.id).to eq(ship.id)
    end

    it 'returns ship with correct name' do
      expect(subject.name).to eq(ship.name)
    end
  end
end

02.rb

# Example Two: spec/unit/lib/space/flight/travel_spec.rb

RSpec.describe Space::Flight::Travel do
  let(:ship) { FactoryBot.create(:ship) }
  let(:new_location) { FactoryBot.create(:location) }
  let(:ship_gateway) { Ship }
  let(:use_case) { described_class.new(ship_gateway: ship_gateway) }

  context 'when travelling to new location' do
    subject { use_case.travel(ship.id, to: new_location) }

    it 'updates ships location' do
      expect{subject}.to change{ship.location}.to(new_location)
    end
  end
end

03.rb

# Example Three: spec/unit/lib/space/flight/ship_gateway_spec.rb

RSpec.describe Space::Flight::ShipGateway do
  let(:ship) { FactoryBot.create(:ship) }
  let(:gateway) { described_class.new(ship_repository: Ship) }

  context 'when finding a ship by id' do
    subject { gateway.find_by_id(ship.id) }

    it 'returns ship' do
      expect(subject).to eq(ship)
    end
  end
end

04.rb

# spec/unit/lib/space/flight/ship_gateway_spec.rb

RSpec.describe Space::Flight::ShipGateway do
  let(:ship) { instance_double('Ship', id: 1) }
  let(:ship_class) { class_double('Ship', find_by: ship) }
  let(:gateway) { described_class.new(ship_repository: ship_class) }

  context 'when finding a ship by id' do
    subject { gateway.find_by_id(ship.id) }

    it 'returns ship with correct id' do
      expect(subject.id).to eq(ship.id)
    end

    it 'returns ship with correct name' do
      expect(subject.name).to eq(ship.name)
    end

    it 'retrieves ship from model' do
      subject
      expect(ship_class).to have_received(:find_by)
    end
  end
end

05.rb

# spec/unit/lib/space/flight/travel_spec.rb

RSpec.describe Space::Flight::Travel do
  let(:ship_id) { 1 }
  let(:new_location_id) { 1 }
  let(:ship_gateway) { instance_double('Space::Flight::ShipGateway', update: true) }
  let(:use_case) { described_class.new(ship_gateway: ship_gateway) }

  context 'when travelling to new location' do
    subject { use_case.travel(ship_id, to: new_location_id) }

    it 'updates ships location' do
      subject
      expect(ship_gateway).to have_received(:update).with(ship_id, location_id: new_location_id)
    end
  end
end

06.rb

# lib/space/flight/ship.rb

module Space
  module Flight
    Ship = Struct.new(:id, :name)
  end
end

# lib/space/flight/ship_gateway.rb

require_relative 'ship'

module Space
  module Flight
    class ShipGateway
      def initialize(ship_repository:)
        @ship_repository = ship_repository
      end

      def find(ship_id)
        ship_record = ship_repository.find_by(id: ship_id)
        build_ship(ship_record) unless ship_record.nil?
      end

      private

      def build_ship(ship_record)
        Ship.new(
          ship_record.id,
          ship_record.name
        )
      end
    end
  end
end