photo_album.rb

  
  def set_cover(should_be_cover, photo)
    if should_be_cover
      cover = photo
    elsif cover.nil?
      # set album cover to first photo album if no cover has been set.
      cover = photos.first
    end
  end
  

photo_spec.rb

require 'spec_helper'
 
describe PhotoAlbum do

    before(:each) do
      @photo_album = stub_model(PhotoAlbum, :id => 1)
      @photo = stub_model(Photo, :photo_album_id => @photo_album.id)
    end
 
   describe 'saving a photo' do
 
    it "should set the cover of the album to the curent photo when 'album cover' is checked" do
      @photo_album.set_cover(true, @photo)
      @photo_album.cover.should == @photo
    end
    
    it "should set the cover to the album's first photo when there are multiple photos, and there's currently no cover set and 'album cover' is not checked" do
      5.times do
        @photo_album << stub_model(Photo, :photo_album_id => @photo_album.id)
      end
      
      @photo_album.set_cover(false, @photo)
      @photo_album.cover.should_not == @photo
    end
    
    it "should set the cover to the the photo if there are no other photos in the album" do
      @photo_album.set_cover(false, @photo)
      @photo_album.cover.should == @photo
    end
  end
 
end

photos_controller.rb

  def create
    @photo = Photo.new(params[:id])
    
    if @photo.save && @photo.photo_album.set_cover(params[:photo_album_cover], @photo)
      redirect_to photos_path
    else
      render :new
    end
  end