Question about using ".build" in association with has_one and belongs_to

hi, guys,

I have the following db tables:
1) Addresses - represents an address (ie line 1, line 2,
suburb,postcode, state and country)
2) Accounts - represents a user account. Will have a foreign key,
"address_id"
3) Warehouses - represents a warehouse. Will have a foreign key,
"address_id"

Each Account object can have one or zero (none) corresponding Address
object.
Each Warehouse object can have one or zero (none) corresponding
Address object.

I started off reading http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html
and http://guides.rubyonrails.org/getting_started.html#associating-models
on the ".build" helper.

From what I understand in context of the latter url, the model
assigned with the "belongs_to" will have a foreign key attribute.

In my case, consider that a new Account is being created (via
registration).
The user goes to his/her profile and clicks on the "create new
Address" to fill in his/her address for the account.

The Addresses controller's 'new' method is called.
This is where I am stuck.

If i did something like

def new
    @address = @account.address.build
end

this will cause a new Addresses object to be built with a non-existent
attribute, :user_id to be set with the new address' id.

An option I have considered is to set the address id value into the
Account object's foreign key, ':address_id' when
a new address is being submitted for creation (ie. calling Addresses
controller method, 'create').
Here's an example:

    def create
        @address = Address.new(params[:address])

        if @address.save
           @user.id = @address.id
           @user.save
           flash[:notice] = "Account registered!"
           redirect_back_or_default account_url
        else
          render :action => :new
        end
    end

Is there a better way to set the foreign key of an object when an
optionally associated object is being instantiated and saved to the
db?

thanks