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

in first rails app i'm trying use form_for , fields_for create nested object form. far good, can't figure out how access sub-object while in fields_for block. i've pre-populated field in sub-object data want show in 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   in 1..5      @garage.cars.build :stall_number =>   end end 

_form.html.erb

<%= form_for @garage |f| %>   <%= f.label :title, "garage name" %><br />   <%= f.text_field :title %>   <% f.fields_for :cars |builder| %>     <p>enter license car parked in stall: <%= car.stall_number %></p>     <%= f.label :license, "license #:" %><br />     <%= f.text_field :license %>   <%= end %> <%= end %> 

as can see, inside builder block :cars, want show, in user instructions, field: car.stall_number (populated in controller integer):

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

i've tried many different ideas: @car.stall_number, object.car.stall_number, etc. no joy. multiple searches , @ fields_for source code haven't helped understanding. appreciate guidance.

update: clarification, per dan's suggestion have tried builder.stall_number results in a

nomethoderror: undefined method 'stall_number' #<actionview::helpers::formbuilder:0x00000102a1baf0> 

i dealt today myself.

you can access object of fields_for through:

builder.object 

where builder fields_for form builder object. in particular case, can say:

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

that should you!