photos.html.erb

<ul class="photos">
    <% @album.photos.in_groups_of(20, false) do |photos| %>
      <div class="slide">
        <% photos.each do |photo| %>          
          <li class="photo">
            <%= link_to image_tag(photo.source(:square)), photo.source(:large), rel: 'gallery', class: 'fancybox-thumb' %>
          </li>
        <% end %>
      </div>
    <% end %>
  </ul>

photos.html.haml

%ul.photos
  - @album.photos.in_groups_of(20, false) do |photos|
    .slide
      - photos.each do |photo|
        %li.photo
          = link_to image_tag(photo.source(:square)), photo.source(:large), rel: 'gallery', class: 'fancybox-thumb'