I'm trying to create categories that have their own subcategories, and each subcategory may have its own subcategories, .. and so on.

After some reading, I found out that the best way for creating such a thing is to use acts_as_tree, but I didn't find any tutorial or article that explains this in a clear way. After a lot of work I was finally able to create categories and subcategories using the following code explained in (Agile Web Development With Rails) book:

root = Category.create(:name => "Books")
fiction = root.children.create(:name => "Fiction")
non_fiction = root.children.create(:name => "Non Fiction")
non_fiction.children.create(:name => "Computers")
non_fiction.children.create(:name => "Science")
non_fiction.children.create(:name => "Art History")
fiction.children.create(:name => "Mystery")
fiction.children.create(:name => "Romance")
fiction.children.create(:name => "Science Fiction")
And here is the database scheme:

create table categories (
id int not null auto_increment,
name varchar(100) not null,
parent_id int,
constraint fk_category foreign key (parent_id) references categories(id),
primary key (id)
In the model:
class Category < ActiveRecord::Base
acts_as_tree :order => "name"
OK, my problem now is how to view something! Any category name, or any subcategory or a list of categories, I tried a lot of things, like (in the view):

<% for category in @categories %>
    <li><%= link_to h(category.title), :action => 'edit', :id => category %>
        <%= find_all_subcategories(category) %>
<% end %>
But I ALWAYS and whatever I try I got this error message:

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occured while evaluating nil.each

Extracted source (around line #2): <% for category in @categories %>
What do you think the problem is?
If you know some tutorial that fully explain how to use acts_as_tree to add and show categories that'll be great.