inverse_of for in memory association access

gistfile1.rb

class Shipment < ActiveRecord::Base
  has_many :labels
  accepts_nested_attributes_for :labels
end

class Label < ActiveRecord::Base
  belongs_to :shipment
end

ruby-1.9.2-p180 :001 > s = Shipment.new(:vendor_service_code =>'asdasd', "labels_attributes"=>{"0"=>{"package_weight"=>"1", "insured_value"=>"50"}})
 => #<Shipment id: nil, user_id: nil, state: "processing", ref_1: nil, ref_2: nil, po_number: nil, total_weight: 1.0, package_count: 1, residential: false, return_label: false, vendor_service_code: "asdasd", vendor_error_message: nil, ship_at: nil, created_at: nil, updated_at: nil, no_courier: false, vendor_package_code: "YOUR_PACKAGING", insured_value: 50, cod: nil, cod_amount: nil, cod_type: nil> 
ruby-1.9.2-p180 :002 > s.labels.first.shipment.vendor_service_code
NoMethodError: undefined method 'vendor_service_code' for nil:NilClass


*** Ahhh inverse_of.....


class Shipment < ActiveRecord::Base
  has_many :labels, :inverse_of => :shipment
  accepts_nested_attributes_for :labels
end

class Label < ActiveRecord::Base
  belongs_to :shipment, :inverse_of => :label
end

ruby-1.9.2-p180 :013 > s = Shipment.new(:vendor_service_code =>'asdasd', "labels_attributes"=>{"0"=>{"package_weight"=>"1", "insured_value"=>"50"}})
 => #<Shipment id: nil, user_id: nil, state: "processing", ref_1: nil, ref_2: nil, po_number: nil, total_weight: 1.0, package_count: 1, residential: false, return_label: false, vendor_service_code: "asdasd", vendor_error_message: nil, ship_at: nil, created_at: nil, updated_at: nil, no_courier: false, vendor_package_code: "YOUR_PACKAGING", insured_value: 50, cod: nil, cod_amount: nil, cod_type: nil> 
ruby-1.9.2-p180 :014 > s.labels.first.shipment.vendor_service_code
 => "asdasd" 
ruby-1.9.2-p180 :015 >