help with nested creation: what am I doing wrong?

I've got nested scaffolded resource objects as indicated below. All
the routing and display stuff seems to work properly, but when I
create a new identity, it shows up in the database with a NULL
user_id, even though I'm creating the object via the identities
attribute of a user object. What am I missing?

Relevant snippets:

config/routes.rb:
map.resources :users do |users|
     users.resources :identities
end

models/user.rb:
class User < ActiveRecord::Base
    has_many :identities
end

models/identity.rb:
class Identity < ActiveRecord::Base
    belongs_to :user
end

controllers/identities_controller.rb:
class IdentitiesController < ApplicationController
    before_filter :get_user

   def get_user
      @user = User.find(params[:user_id])
   end

  def create
    @identity = @user.identities.new(params[:identity])
    respond_to do |format|
       if @identity.save
         flash[:notice] = 'Identity was successfully created.'
         format.html { redirect_to identity_url(@user, @identity) }
         format.xml { head :created, :location =>
identity_url(@user,
                                  @identity) }
       else
         format.html { render :action => "new" }
         format.xml { render :xml => @identity.errors.to_xml }
       end
    end
  end

end

try changing
@identity = @user.identities.new(params[:identity])

to
@identity = @user.identities.build(params[:identity])