## new.html.haml
%h1 New topic

- form_for(@topic) do |f|
  = f.error_messages

  %p
    = f.label :name
    %br
    = f.text_field :name

  %p
    = f.label :parent
    %br
    = collection_select(:topic, :parent, Topic.all, :id, :name)
    / The above is the offending line of code
  %p= f.submit 'Create'

= link_to 'Back', topics_path

## topics_controller.rb
  def create
    @topic = Topic.new(params[:topic])
    @parent = Topic.find(params[:parent])
    respond_to do |format|
      if @topic.save
      # ...

## The error [plain_text]
ActiveRecord::UnknownAttributeError in TopicsController#create

unknown attribute: parent

## topic.rb

class Topic < ActiveRecord::Base
  has_many :lessons
  has_many :sections
  has_many :topics, :id => :lft  # should I do something like this to make standard AR association stuff work?
  acts_as_nested_set
end