how to have checkboxes in the view for a many-to-many relationship

controller.rb

# in controller:

  def update

    # (.....)

    @categories = Category.all
    checked_categories = []
    checked_params = params[:category_list] || []

    for check_box_id in checked_params
      category = Category.find(check_box_id)
      if not @texture.categories.include?(category)
        @texture.categories << category
      end
      checked_categories << category
    end
    missing_categories = @categories - checked_categories
    for category in missing_categories
      if @texture.categories.include?(category)
         @texture.categories.delete(category)
       end
    end

    # (.....)

  end

model: category.rb

class Category < ActiveRecord::Base
  has_and_belongs_to_many :textures
end

model: texture.rb

class Texture < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

rails checkboxes for many_to_many.html.erb

<% # in view: %>

<% form_for(@texture) do |f| %>

 <% #(.....) %>

  <% for category in @categories %>
  	<%= check_box_tag(
  	  'category_list[]', 
  	  category.id, 
  	  @texture.categories.collect {|obj| obj.id}.include?(category.id)) 
  	%>
  	<%= category.name %><br />
  <% end %>

 <% #(.....) %>

<% end %>