HABTM through editing via checkboxes

I have a HABTM relationship between Publications and Categories. In the new and edit views, I have this:

Categories:<br />
<% @categories.each do |c| %>
  &nbsp;<%= check_box_tag :category_ids, c.id, @publication.categories.include?(c), :name => 'publication[cate开发者_如何学JAVAgory_ids]' -%>
  <%= "#{c.name}"%><br />
<% end -%>

The model code:

class Publication < ActiveRecord::Base
   has_many :listings
   has_many :categories, :through => :listings, :order => "listings.position"

This displays just fine - on update, though, it only saves the last category that is checked (if I check multiple categories, only one is saved), if I don't check any box, it doesn't change what's mapped.

---

My problem - I had to implement a category_ids setter on Publication. I implemented it as so:

after_save :update_categories  
  attr_accessor :category_ids

  def update_categories
    unless category_ids.nil?
      self.listings.each do |listing|
        listing.destroy unless category_ids.include?(listing.category_id.to_s)
        category_ids.delete(listing.category_id.to_s)
      end 
      category_ids.each do |cid|
        self.listings.create(:category_id => cid) unless cid.blank?
      end
      reload
      self.category_ids = nil
    end
  end