photo_claimer.rb

require 'upload_store'

class PhotoClaimer

  class FileNotFound < RuntimeError; end

  def initialize(user_id, file_name)
    @user_id    = user_id
    @file_name  = file_name
  end

  # Public: Do it!

  # Returns a Photo instance.
  def claim
    photo = create_record

    delete_file

    photo
  end

  private

  def create_record
    photo = Photo.create!(user_id: @user_id)

    # parent model must have identity before saving because the id
    # is used for the file path.
    photo.image.store!(tempfile)
    photo.save!
  end

  def tempfile
    tmp = Tempfile.new(@file_name)
    tmp.write(uploaded_file.body)
    tmp.rewind
    tmp
  end

  def delete_file
    uploaded_file.destroy
  end

  # Internal: Find the associated uploaded file.
  #
  # Returns: Fog::File instance.
  # Raises FileNotFound error when file cannot be located.
  def uploaded_file
    @uploaded_file ||= UploadStore.get(@file_name)

    raise FileNotFound, "#{@file_name} not found" unless @uploaded_file

    @uploaded_file
  end
end

photo_claimer_spec.rb

require_relative '../../../app/domain/photo_claimer'

describe PhotoClaimer do
  let(:claimer) { PhotoClaimer.new(1, 'some/path.jpeg') }

  before do
    stub_const('Photo', double(:photo_class).as_null_object)
    stub_const('Tempfile', double(:tempfile_class).as_null_object)
    stub_const('UploadStore', double(:uploadstore_class).as_null_object)
  end

  it 'errors when file cannot be found' do
    UploadStore.stub(:get) { nil }
    expect{ claimer.claim }.to raise_error(PhotoClaimer::FileNotFound)
  end

  it 'deletes the uploaded file' do
    file = double(:file).as_null_object
    UploadStore.stub(:get) { file }

    file.should_receive(:destroy)
    claimer.claim
  end

  it 'creates a photo record' do
    Photo.should_receive(:create!)
    claimer.claim
  end

  it 'assigns attaches uploaded file' do
    tempfile  = double(:tempfile).as_null_object
    uploader  = double(:uploader)
    photo     = double(:photo, image: uploader)

    Photo.stub!(:create!) { photo }
    Tempfile.stub(:new) { tempfile }

    uploader.should_receive(:store!).with(tempfile)
    photo.should_receive(:save!)

    claimer.claim
  end
end

Photo#image is a Carrierwave uploader, ImageUploader. This class depends on Photo object with a unique id such that it can know where to store the files. There are two options here. We can ignore the coupling problem illustrated in our spec, 'assigns attaches uploaded file...', or we can find a way to create a new unique id which is used to create a storage location.