Ruby on Rails

i'm trying to follow these instructions:

in my categories_controller.rb

def index

@parent_categories = Category.without_parents

in my index.html.erb

<ul>

<%= render @parent_categories %>

</ul>

in my _categories.html.erb

<ul>

<li>

<strong><%= category.name %></strong>

<ul>

<%= render category.categories %>

</ul>

</li>

</ul>

but this doesn't work i get unknown method wihtout_parents and couldn't figure out what without_parents was supposed to be so, i tried changing the line in the controller to @parent_categories = Category.where("parent_id=0") and it was on to the next error, now i get undefined method `categories' for #<Category:0xb1604e2c> My understanding is that in the statement, category is the partial view and categories means the entire collection, so then why doesn't rails recognize the meaning of categories as the entire collection and what's the story with "without_parents" ? ~ thanks, fugee