many-to-many.rb

# categorization.rb

class Categorization < ActiveRecord::Base
  attr_accessible :category_id, :post_id
  belongs_to  :category
  belongs_to  :post
end

# category.rb

class Category < ActiveRecord::Base
  attr_accessible :name
  has_many  :categorizations
  has_many  :posts, :through => :categorizations
end

# post.rb

class Post < ActiveRecord::Base
  attr_accessible :title, :category_ids
  has_many  :categorizations
  has_many  :categories, :through => :categorizations
end

# view.html.erb

<div class="field">
    <% Category.all.each do |c| %>
      <br><%= check_box_tag 'post[category_ids][]', c.id,
                            @post.categories.include?(c), :id => "post_category_ids_#{c.id}" %> <%= label_tag "post_category_ids_#{c.id}", c.name %>
    <% end %>
  </div>