Rails 3.1: Nested attributes won't save through the form - ruby-on-rails

Question

I suspect this might be a very simple mistake but I've spent 3 hours looking for it so I thought I might ask for some help from the community.
I'm running through Ryan Bates' excellent screencasts on Nested Models Forms and trying to apply them to my own project. The problem is the nested attribute doesn't seem to save using the form. I can get it to save through the console but it only shows up as empty brackets when going through the form.
Here's the relevant code:
The form view (using haml)
= form_for(#article) do |f|
- if #article.errors.any?
#error_explanation
%h2
= pluralize(#article.errors.count, "error")
prohibited this article from being saved:
%ul
- #article.errors.full_messages.each do |msg|
%li= msg
.field
= f.label :title
%br/
= f.text_field :title
.field
= f.label :intro
%br/
= f.text_area :intro
= f.fields_for :subsections do |builder|
= render 'subsections_fields', :f => builder
.field
= f.label :published_at
%br/
= f.text_field :published_at
.actions
= submit_or_cancel(f)
subsection_fields form view
= f.label :header
%br/
= f.text_field :header
= f.label :order_id
= f.number_field :order_id
%br/
= f.label :body
%br/
= f.text_area :body
%br/
= f.check_box :_destroy
= f.label :_destroy, "Remove Subsection"
%br/
Controller
class ArticlesController < ApplicationController
def new
#article = Article.new
3.times { #article.subsections.build }
end
def create
#article = Article.new(params[:article])
if #article.save
flash[:notice] = "Successfully created article."
redirect_to #article
else
render :action => 'new'
end
end
def edit
#article = Article.find(params[:id])
end
def update
#article = Article.find(params[:id])
if #article.update_attributes(params[:article])
flash[:notice] = "Successfully updated article."
redirect_to #survey
else
render :action => 'edit'
end
end
def destroy
Article.find(params[:id]).destroy
flash[:notice] = "Succesfully destroy article."
redirect_to articles_url
end
def show
#article = Article.find(params[:id])
end
def index
#articles = Article.all
end
end
And the models
class Article < ActiveRecord::Base
attr_accessible :title, :intro
has_many :subsections, :dependent => :destroy
accepts_nested_attributes_for :subsections, :reject_if => lambda { |a| a[:body].blank? },
:allow_destroy => true
has_and_belongs_to_many :categories
validates :title, :presence => true
end
class Subsection < ActiveRecord::Base
attr_accessible :header, :body, :order_id
belongs_to :article
validates :header, :presence => true
validates :body, :presence => true
end
Any help figuring this out is much appreciated.

Answer 1

I'm not quite sure, but try it with attr_accessible :article_id as well in your Subsection model?

Answer 2

Adding "attr_accessible" to a model changes the way mass assignment works in rails.
If you remove the "attr_accessible" lines in your models then all your code will work perfectly as it is.
The class method "accepts_nested_attributes_for" adds a "subsections_attributes=(value)" method to your model.
The second you add "attr_accessible" to a model you now are forced into adding extra "attr_accessible" entries for each field that you want to assign via mass assignment. i.e. when you use Article.new(params[:article]).
I hope that was clear.

Answer 3

I figured out the answer to this one from another question.
The answer was to set my subsections_attributes as an attr_accessible, so the above Article model should look like this (I also added published_at as an attr_accessible):
class Article < ActiveRecord::Base
attr_accessible :title, :intro, :subsections_attributes, :published_at
has_many :subsections, :dependent => :destroy
accepts_nested_attributes_for :subsections, :reject_if => lambda { |a| a[:body].blank? },
:allow_destroy => true
has_and_belongs_to_many :categories
validates :title, :presence => true
end