Three models problem in 'new' action

Hello,

I have this problem :

When I use three models, and associate them, then, my ‘new’ action need
more data to work, How can I resolv it, or I do it in the right way ?

Thanks

Jose.

******************** my models *************************

class Libro < ActiveRecord::Base
has_many :lecciones
end

class Leccion < ActiveRecord::Base
has_many :vocabularios
belongs_to :libro
end

class Vocabulario < ActiveRecord::Base
belongs_to :leccion
belongs_to :libro
end

**************** routes.rb ****************************

ActionController::Routing::Routes.draw do |map|
map.resources :vocabularios

map.resources :lecciones

map.resources :libros

map.resources :libros, :has_many => :lecciones

map.resources :libros do |libro|
libro.resources :lecciones do |leccion|
leccion.resources :vocabularios
end
end

map.connect ‘:controller/:action/:id’
map.connect ‘:controller/:action/:id.:format’
end

*************** my controllers *****************************

class VocabulariosController < ApplicationController

def new
@libro = Libro.find(params[:libro_id])
@leccion = @libro.lecciones.find(params[:leccion_id])
@vocabulario = @leccion.vocabularios.build(params[:vocabulario])

respond_to do |format|
  format.html # new.html.erb
  format.xml  { render :xml => @vocabulario }
end

end

def create
@libro = Libro.find(params[:libro_id])
@leccion = @libro.lecciones.find(params[:leccion_id])
@vocabulario = @leccion.vocabularios.build(params[:vocabulario])

# Here '@vocabulario' never have 'libro' and write in the
# database 'NULL'
# why it never works ?

# i need to put something like :

# @vocabulario.libro = @libro

# then the insert in database works fine, WHY ?
# is this ok ?

if @vocabulario.save
  redirect_to libro_leccion_vocabulario_url(@libro, @leccion, 

@vocabulario)
else
render :action => “new”
end

end

end