listbox with checkboxes in rails

I've been searching for hours without any luck. All I want is a listbox containing checkboxes that correspond to a has_many relation. I have my checkboxes but they are shown as separate checkboxes, not together in a list. I must be missing something simple but can't seem to find it. Please help. I use the code below to show my checkboxes. How can I group them in a listbox?

<%= f.collection_check_boxes :logical_attribute_ids, LogicalAttribute.all, :id, :name do |att| %>
  <%= att.check_box %>
  <%= att.label %>
<% end %>