Rails - Sep 2005 - habtm example

(this code is also at http://rafb.net/paste/results/KPgNpg49.txt )

# This code is sort of cool, I think.  Too bad that I have to write it
for habtm.
#
# This is a fairly easy way to let users edit a habtm relationship with
# checkboxes.
#
# Following is the relevant code for a Product that habtm Categories, Artists,
# Authors, and publishers.


# in views/products/_form.rhtml:
<%= product_habtm_selector Category, @product %>
<%= product_habtm_selector Artist, @product %>
<%= product_habtm_selector Author, @product %>
<%= product_habtm_selector Publisher,@product %>


# in a helper:
  # Creates a table that has checkboxes for a habtm relationship
  # with a product.
  def product_habtm_selector klass, product
    cols = 3
    count = 0
    result = "#{ klass.to_s.pluralize } <br />"
    result << "<table>"
    klass.find(:all).each do |thing|
      result << "<tr>" if count % rows == 0
      result << "<td valign=''top''>"
      result << "<label for=''#{ thing.name
}''>"
      result << check_box_tag("product[#{
klass.to_s.pluralize.downcase }_ids][]",
                              thing.id,
                              product.send("has_#{ klass.to_s.downcase
}?", thing.id))
      result << thing.name
      result << "</label>"
      result << "</td>"
      result << "</tr>" if count % cols == (count - 1)
      count += 1
    end
    result << "</table>"
    result
  end



# Extending active record with a couple methods.
class ActiveRecord::Base
  def self.define_habtm_setter(the_many)
    module_eval <<-CODE
    def #{the_many}_ids=(ids)
      #{the_many}.clear
      for id in ids
        if id.respond_to? :read
          #{the_many} << #{the_many.to_s.classify}.find(id.read)
        else
          #{the_many} << #{the_many.to_s.classify}.find(id)
        end
      end
    end

    validates_associated :#{the_many}

    def has_#{the_many.to_s.singularize}?(id)
      #{the_many}.to_a.find {|a| a.id == id}
    end
    CODE
  end


# Models:
class Product < ActiveRecord::Base
  has_and_belongs_to_many :categories
  define_habtm_setter     :categories
  has_and_belongs_to_many :authors
  define_habtm_setter     :authors
  has_and_belongs_to_many :artists
  define_habtm_setter     :artists
  has_and_belongs_to_many :publishers
  define_habtm_setter     :publishers
  ...
end

class Publisher < ActiveRecord::Base
  has_and_belongs_to_many :products
end
# and Artists, Authors, and Categories are the same as Publisher.

# ProductsController#update
# If no checkboxes are in params, then the user has deselected all checkboxes.
  def update
    @product = Product.find(params[:id])
    if @product.update_attributes(params[:product])
      if params[:product][:categories_ids].nil?; @product.categories.clear; end
      if params[:product][:authors_ids].nil?; @product.authors.clear; end
      if params[:product][:artists_ids].nil?; @product.artists.clear; end
      if params[:product][:publishers_ids].nil?; @product.publishers.clear; end
  ...
end