photo.rb

class Photo

  belongs_to :photo_album

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

end

photo_spec.rb

require 'spec_helper'

describe Photo do
  
  describe 'creating a new photo' do
    
    before(:each) do
      @photo_album = stub_model(PhotoAlbum, :name => 'a photo album', :id => 1)
      @photo = stub_model(Photo, :name => 'a photo', :photo_album_id => @photo_album)
    end
    
    it 'should set the first photo to be the cover of the album if no cover is set' do
      @photo.stub!(:set_cover).with(false)
      @photo.photo_album.cover.should == @photo_album.photos.first
    end
    
    it 'should set the album cover to the current photo' do
      @photo.stub!(:set_cover).with(true)
      @photo.photo_album.cover.should == @photo_album.cover
    end
  end
end