Whats wrong with this form routing? - ruby-on-rails

Question

I have a users model and a book model. Users can read books (as a reader) which creates an entry in the Readings model:
id | reader_id | book_id
Users also have a list of books that they have read. These are stored in the Red (I use Red because the present and past tense of the word 'read' are the same) model which looks the same as the Reading model above.
Now when a user is reading a book, I would like to display a button which represents finishing the book.
The finish action is in the ReadingsController and looks like this:
def finish
#book = current_user.readings.find(params[:id]).book
current_user.stop_reading!(#book)
current_user.make_red! #book
redirect_to :back
end
As you can probably tell, this takes in the id of a record in the readings table, destroys it and makes a new record in the table for recording books red.
The form helper for the "Finish Reading" button currently looks like this:
<%= form_for :reading, current_user.readings.find_by_book_id(book.id), :url => { :controller => :readings, :action => "finish" }, :method => :delete do |f| %>
<div class="actions"><%= f.submit button_text %></div>
<% end %>
But for some reason this renders a form with the wrong id because "9781440506604" is not the id of a record in the readings table, it's the id of a record in the books table (the ISBN-13 of a book to be precise).
<form accept-charset="UTF-8" action="/readings/9781440506604/finish" method="post">
</form>
What is it I'm doing wrong?
EDIT to add reading.rb
class Reading < ActiveRecord::Base
attr_accessible :book_id
# one person reading a new book may cause feed_item creations in multiple users feeds
has_many :feed_items, :as => :event
has_many :comments, :as => :parent, :dependent => :destroy
scope :from_users_followed_by, lambda { |user| followed_by(user) }
# need to pass the class name here because there is no Reader model
belongs_to :reader, :class_name => "User"
belongs_to :book
validates :reader_id, :presence => true
validates :book_id, :presence => true
def self.followed_by(user)
...
end
end
# and user.rb
class User < ActiveRecord::Base
attr_accessible :name, :email, :password, :password_confirmation, :avatar, :remember_me, :avatar_url
has_many :readings, :dependent => :destroy,
:foreign_key => "reader_id"
has_many :reads, :through => :readings, :source => :book
has_many :reds, :foreign_key => "reader_id",
:dependent => :destroy
has_many :red, :through => :reds, :source => :book
def reading? book
self.readings.find_by_book_id(book)
end
def read! book
self.readings.create!(:book_id => book.id)
end
def stop_reading! book
self.readings.find_by_book_id(book).destroy
end
def red? book
self.reds.find_by_book_id(book)
end
def make_red! book
unless red? book
self.reds.create!(:book_id => book.id)
end
end
end
By the way I tried making a user who is reading book 1 and doing user.readings.find_by_book_id(1) in the console and it returns a record from the readings table.
as requested
# routes.rb
resources :readings, :only => [:create, :destroy, :show] do
member do
post :create_comment
delete :finish
end
end

Answer 1

Looks like you have got to_param method in your Reading model
try to call id clearly:
current_user.readings.find_by_book_id(book.id).id
UPD
remove :only => [:create, :destroy, :show] from your routes
use this <%= form_for :reading, current_user.readings.find_by_book_id(book.id), :url => { :controller => :readings, :action => "finish", :id => current_user.readings.find_by_book_id(book.id).id }, :html => {:method => :delete} do |f| %>

Answer 2

I'm not particularly knowledgeable about rails 3 (still using rails 2), but shouldn't you be passing more information to the :url param?
This doesn't seem to mention anything about the ID you want to post to:
:url => { :controller => :readings, :action => "finish" }
Shouldn't it be something closer to this:
:url => { :controller => :readings, :action => "finish", :id => reading_id }
(Assuming reading_id to be substituted for the actual ID)