Rails - Using form_for and fields_for, how do you access the sub-object while in the fields_for block?

Question

In my first rails app I'm trying to use form_for and fields_for to create a nested object form. So far so good, but I can't figure out how to access the sub-object while in the fields_for block. I've pre-populated a field in the sub-object with data that I want to show in the user instructions.

Models
Garage:

has_many :cars, :dependent => :destroy         
accepts_nested_attributes_for :cars

Car:

belongs_to :garage

Garage Controller

def new
  @garage = Garage.new
  for i in 1..5 
    @garage.cars.build :stall_number => i
  end
end

_form.html.erb

<%= form_for @garage do |f| %>
  <%= f.label :title, "Garage Name" %><br />
  <%= f.text_field :title %>
  <% f.fields_for :cars do |builder| %>
    <p>Enter license for car parked in stall: <%= car.stall_number %></p>
    <%= f.label :license, "License #:" %><br />
    <%= f.text_field :license %>
  <%= end %>
<%= end %>

As you can see, inside the builder block for :cars, I want to show, in my user instructions, the field: car.stall_number (populated in my controller with an integer):

<p>Enter license for car parked in stall: <%= car.stall_number%></p>

I've tried a many different ideas: @car.stall_number, object.car.stall_number, etc. No joy. Multiple searches and a look at the fields_for source code haven't helped my understanding. I would appreciate any guidance.

Update: For clarification, per Dan's suggestion I have tried builder.stall_number but it results in a

NoMethodError: undefined method 'stall_number' for #<ActionView::Helpers::FormBuilder:0x00000102a1baf0>

Answer 1

I just dealt with this today myself.

You can access the object of the fields_for through:

builder.object

where builder is your fields_for form builder object. In your particular case, you can say:

<p>Enter license for car parked in stall: <%= builder.object.stall_number%></p>

That should do it for you!

Answer 2

The way you are trying is does not work because you want to access car without filling that variable for data.

I guess you want to have multiple blocks of stalls, where you can enter license plates. For each stall you will need your own fields_for. I would suggest something like that:

<%= form_for @garage do |f| %>
  <%= f.label :title, "Garage Name" %><br />
  <%= f.text_field :title %>

  <% for i in 1..5 %>
    <% f.fields_for @garage.cars[i] do |builder| %>
      <p>Enter license for car parked in stall: <%= builder.stall_number%></p>
      <%= builder.label :license, "License #:" %><br />
      <%= builder.text_field :license %>
    <% end %>
  <% end %>
<% end %>

Within the fields_for you need to use the form object you define there, in this case builder. Since the data there are not mapped to the outer form (f), but to the cars object (builder).