Mongoid many-to-many relationship problem adding a record - ruby-on-rails-3

Question

I'm using Mongoid/MongoDB with Rails and am trying to get a many-to-many relationship working. Basically books and categories where books can be in multiple categories. I keep getting an error:
undefined method `metadata' for "4e6aaec8ffb1900c19000002":String
when trying to add a new book and place it in categories. The following is what I'm using for the models, form, create method and what the server is reporting.
It looks like it is trying to update book_ids and the cat_ids, but it's not getting anything for the cat_ids. I've been trying lots of different things, but am not sure how to make this work.
The book model
class Book
include Mongoid::Document
field :title, :type => String
field :description, :type => String
has_and_belongs_to_many :cats
end
The cat model (categories)
class Cat
include Mongoid::Document
field :name, :type => String
field :description, :type => String
has_and_belongs_to_many :books
end
This is from the form that generates the category select and allows multiple selections:
<div class="field">
<%= label_tag "Cats" %><br />
<%= f.collection_select :cats, Cat.all, :id, :name, {}, :multiple => true %>
</div>
The create method in the books controller:
def create
#book = Book.new(params[:book])
redirect_to(#book, :notice => 'Book was successfully created.')
end
From the server when submitting the form:
Started POST "/books" for 127.0.0.1 at Fri Sep 09 17:30:37 -0700 2011
Processing by BooksController#create as HTML
Parameters: {"commit"=>"Create Book", "authenticity_token"=>"+OAIJM3NRPrUv0u1yfDEkkE2gvPQ7n0P6zPU9ZtqXlk=",
"utf8"=>"✓", "book"=>{"title"=>"The Golf & Tennis Book",
"cats"=>["4e6aaec8ffb1900c19000002", "4e6aaee8ffb1900c19000006"],
"description"=>"Both golf and tennis in this book, so it's in both categories."}}
MONGODB blog_development['system.namespaces'].find({})
MONGODB blog_development['cats'].update({:_id=>{"$in"=>[]}}, {"$pull"=>{"book_ids"=>BSON::ObjectId('4e6aafadffb1900c1900000b')}})
MONGODB blog_development['system.namespaces'].find({})
MONGODB blog_development['books'].update({"_id"=>BSON::ObjectId('4e6aafadffb1900c1900000b')}, {"$set"=>{"cat_ids"=>[]}})
Completed 500 Internal Server Error in 24ms
NoMethodError (undefined method `metadata' for "4e6aaec8ffb1900c19000002":String):
app/controllers/books_controller.rb:46:in `new'
app/controllers/books_controller.rb:46:in `create'

Answer 1

This is invalid
book.update_attributes({:cats => [ cat.id ]})
should be this
book.update_attributes({:cat_ids => [ cat.id ]})
or
book.update_attributes({:cats => [ cat ]})