update values of checkbox with HABTM relationship -- Rails

Hey guys I've been using the has_and_belongs_to_many relationship with checkboxes example from the Railscast Episode #17 . I had some problems and now everything is working kind of smoothly except the update button will not work.

the edit view looks like so

<% form_for :users, :action => 'update'  do |f| %>
<% for interest in Interest.find(:all) %>
<div>
<%= check_box_tag "user[interest_ids][]", interest.id,
              @user.interests.include?(interest) %>
<%= interest.na开发者_如何学Cme %>
</div>
<% end %>
<p>
<%= f.submit 'Update' %>
</p>
<% end %>

and in the controller I have ....

def edit
  @user = User.find(session[:user_id])
end

 def update
  params[:user][:interest_ids] ||= []
  @user = User.find(session[:user_id])
  if @user.update_attributes(params[:user])
  flash[:notice]='User data was updated'
  redirect_to :action => 'index'
 else
  redirect_to :action => 'index'
 end  
end

The button isn't event doing the redirect... so I dont know whats happening. Is there something in my form creation messing this up? I'm not exactly sure how to create an button and have it access the method in the controller with model updates and etc.

I looked around for help and thought maybe it was because of attr_accessible so I added =>

 attr_accessible :login, :email, :password, :password_confirmation, :interest_ids, :user

to my user model but still nothing...Any I idea why my form isn't submitting?

---

I played around with the form and the correct code is this ==>

<% form_for @user do |f| %>
<% for interest in Interest.find(:all) %>
<div>
 <%= check_box_tag "user[interest_ids][]", interest.id, @user.interests.include?(interest) %>
 <%= interest.name %>
</div>
<% end %>
<p>
<%= f.submit 'Edit' %>
</p>
<% end %>

I'm so happy! So apparently the problem was :user which should the be object @user.

---

The check_box tag must look like this for it to work:

<%= check_box "user", "user_interest_ids", {:checked => @user.interests.include?(interest), :name => 'user[interest_ids][]'}, interest.id, nil %>

In my ad <-> ad_sizes habtm relationship I'm using this code:

<% @ad_sizes.each do |s| %>
<li>
<%= check_box "ad", "ad_size_ids", {:checked => @ad.ad_sizes.include?(s), :id => "ad_size_#{s.name}", :class => 'checkbox', :name => 'ad[ad_size_ids][]'}, s.id, nil %>
<label class="checkbox" for="<%= "ad_size_#{s.name}" %>"><%= s.description %></label>
</li>
<% end %>