has_one with nested form

I have an Item model with has_one :geolocation association I need to
create/update in one form

class Item < ActiveRecord::Base
  has_one :geolocation, :dependent => :destroy
  accepts_nested_attributes_for :geolocation, :reject_if
=> :all_blank, :allow_destroy => true
  attr_accessible :geolocation_attributes
..
end

class Geolocation < ActiveRecord::Base
    belongs_to :item
..
end

as stated in all docs
in my Items table migration , I have a geolocation_id ( has_one
association )
and in my Geolocations table , I have an item_id ( belongs_to
association )

Creating the item in the controller
  def new
    @item = Item.new
    @item.geolocation = Geolocation.new
    respond_with(@item)
  end
  def create
    @item = Item.new(params[:item])
    if @item.save
      redirect_to backoffice_item_path(@item), notice:
t(:item_created)
    else
      render :new
    end
  end

with a nested form
...
= simple_form_for @item, :url => backoffice_items_path, :html =>
{:class => 'form-vertical' } do |f|
  ...
   = f.fields_for :geolocation do |geolocation|

inserts correctly the item record and the geolocation record BUT in
the item record, the geolocation_id is NOT set ..
as if that the has_one association was not taken in account...

I'm surely missing something important , but I don't see what ...
feedback and cue will be a big +++

thanks

[SOLVED] doesn't need this key ... has_one .. the key is in the
target model 'item_id' in Geolocation