When you have a model that acts_as_tree, you might want to use that to generate a nice select element with option groups.

In your view, simply use:

<%= f.select (:category_id, option_groups_from_collection_for_select(
    Category.top,
    :children, :name, 
    :id, :name
    )) %>

and add a method to your model:

def self.top
  find(:all, :conditions => [ 'parent_id IS NULL' ])
end 

This generates HTML like this:

<select id="product_category_id" name="product[category_id]">
  <optgroup label="Europe">
    <option selected="selected" value="8">France</option>
    <option value="9">Spain</option>
  </optgroup>
  <optgroup label="America">
    <option value="11">USA</option>
    <option value="12">Canada</option>
  </optgroup>
</select>

And it looks like this: France Spain USA Canada