// This takes a collection of acts_as_tree objects and creates a unordered list.
//
// Put this in your helper:

  def tree_ul(acts_as_tree_set, init=true, &block)
    if acts_as_tree_set.size > 0
      ret = '<ul>'
      acts_as_tree_set.collect do |item|
        next if item.parent_id && init
        ret += '<li>'
        ret += yield item
        ret += tree_ul(item.children, false, &block) if item.children.size > 0
        ret += '</li>'
      end
      ret += '</ul>'
    end
  end

// ... and this in your view:

  <%= tree_ul(@acts_as_list_collection) {|item| link_to h(item.name), :action => 'edit', :id => item } %>

You should have a categories table with a parent_id field. Root categories should have the parent_id of 0.

I modified this post:
http://www.bigbold.com/snippets/posts/show/296

because the first one worked if you did stuff in your view to make the root categories come up, but I agree with the commenter that things should be contained to the helper - however I've managed to break the DRY rule in doing so. If anyone has any suggestions for a better way around this I'd love to hear it, so please comment. So anyway.. this should list your root categories ONLY and NOT your other sub-cats as well so that you have a very pretty little tree.

Slap this in your view:

<%= display_categories(@categories) %>

And put this in a helper file. You'll need it in the application helper most likely since you'll want to call it from multiple views.

      def display_categories(categories)
	    	   ret = "<ul>"
	   for category in categories
		   if category.parent_id == 0
			ret += "<li>"
			ret += link_to category.name
			ret += find_all_subcategories(category)
			ret += "</li>"
		   end
		   
	   end
	   ret += "</ul>"
    end
    
   def find_all_subcategories(category)
    if category.children.size > 0
      ret = '<ul>'
      category.children.each { |subcat| 
        if subcat.children.size > 0
          ret += '<li>'
          ret += link_to h(subcat.name), :action => 'edit', :id => subcat
          ret += find_all_subcategories(subcat)
          ret += '</li>'
        else
          ret += '<li>'
          ret += link_to h(subcat.name), :action => 'edit', :id => subcat 
          ret += '</li>'
        end
        }
      ret += '</ul>'
    end
  end

In your view:

<ul>  
<% for category in @categories %>
    <li><%= link_to h(category.title), :action => 'edit', :id => category %>
        <%= find_all_subcategories(category) %>
   </li>
<% end %>
</ul>

Place this in a helper:

  def find_all_subcategories(category)
    if category.children.size > 0
      ret = '<ul>'
      category.children.each { |subcat| 
        if subcat.children.size > 0
          ret += '<li>'
          ret += link_to h(subcat.title), :action => 'edit', :id => subcat
          ret += find_all_subcategories(subcat)
          ret += '</li>'
        else
          ret += '<li>'
          ret += link_to h(subcat.title), :action => 'edit', :id => subcat 
          ret += '</li>'
        end
        }
      ret += '</ul>'
    end
  end