Ruby on Rails

As ever, Colin's advice is correct. Do work through a full tutorial,
maybe two or three times, even.

Notwithstanding that, there are a couple things you're trying to do
here.

1. defined scopes

the `.without_parents` would be a *scope* you have to define as a Class
Method on your Category model. It's essentially like so:

    class Category < ActiveRecord::Base
      # ...

      scope :without_parents, ->() { where( parent_id: 0 ) }

      # ...
    end

See http://guides.rubyonrails.org/active_record_querying.html#scopes


2. partials for collections


See http://guides.rubyonrails.org/layouts_and_rendering.html#using-partials
subsection 3.4.5 Rendering Collections. When you have a collection of
categories, as you do with `@parent_categories`, you name the partial
`category.html.erb` (NOTE SINGULAR) and the variable available inside
that partial is also `category` (again singular) and it contains ONE
category from `@parent_categories` at a time.


--
Tamara Temple
tam...@gmail.com
http://www.tamouse.org