Creating a form based on a :has_many through relationship - ruby-on-rails

Question

I am new to ruby and I am trying to put together a form that will allow you to add items to an order. The form will need to take a quantity for each item in the order.
class Order < ActiveRecord::Base
belongs_to :restaurant
has_many :selections
has_many :items, :through =>:selections;
end
class Selection < ActiveRecord::Base
belongs_to :order
belongs_to :item
end
class Item < ActiveRecord::Base
belongs_to :menu
has_many :selections
has_many :orders, :through => :selections
end
class Restaurant < ActiveRecord::Base
has_many :orders
has_many :menus
has_many :items, :through => :menus
end
class Menu < ActiveRecord::Base
belongs_to :restaurant
has_many :items
end
order controller
# GET /orders/new
def new
#order = Order.new
#restaurant.items.all.each do |item|
#order.selections.build
end
end
orders/_form.html.erb :
The form is supposed to list out the available items and allow you to enter the quantity for an item.
<%= form_for [#restaurant,#order], :html => { :class => 'form-horizontal' } do |f| %>
<div class="control-group">
<%= f.label :current_table, :class => 'control-label' %>
<div class="controls">
<%= f.text_field :current_table, :class => 'text_field' %>
</div>
</div>
<% f.fields_for :selections do |ff| %>
<div class="control-group">
<%= ff.label :quantity, :class => 'control-label' %>
<div class="controls">
<%= ff.text_field :quantity, :class => 'text_field' %>
</div>
</div>
<% end%>
<div class="form-actions">
<%= f.submit nil, :class => 'btn btn-primary' %>
<%= link_to t('.cancel', :default => t("helpers.links.cancel")),
course_orders_path, :class => 'btn' %>
</div>
<% end %>
When I try to render the page I get the following error:
undefined method `quantity' for
<ActiveRecord::Associations::CollectionProxy::ActiveRecord_Associations_CollectionProxy_Selection:0x007ffa0287cd60>
I realize this is probably because I haven't initialized any selections but I'm not entirely sure how/where I should do that. When the form is submitted I want to create an order with a selection for each of the non-empty quantities.
So my first question is how do I construct my form so that I can take a quantity for each item that I know about?
Do I need to initialize anything in the order controller to make this work?
Can you give me any advice or point me to a tutorial that shows me how to set up the create method of the order controller?
EDIT:
I added some code to the Order controller and the form, so when I render the page I no longer get an error, but none of my 'selection' fields rendered. I confirmed with some logging and the debugger that I correctly 'build' 4 selections so I would expect those form elements show up.
Any ideas would be appreciated.

Answer 1

Are you missing a
accepts_nested_attributes_for :nested_class
somewhere?
Also, I had to do something like
<%= form_for [#restaurant,#order] do |f| %>
...
<%= f.fields_for :selections, #order.selections do |ff| %>
...
<% end %>
...
<% end %>