_bags.erb

<div id="bags">
  <ul>
    <% last_store_name = nil %>
    <% bags.each_with_index do |bag, index| %>
      <%
        extra_class = if last_store_name != bag.store.name
          last_store = bag.store
          "first_of_serie #{bag.store.name}"
        else
          ''
        end
      %>
      <li data-index="<%= index %>" class="<%= extra_class %>" data-detail-path="<%= bag.detail_path %>">
        <%= image_tag bag.thumbnail_path %>
      </li>
    <% end %>
  </ul>
</div>