Many to Many example refer to : http://hadiyahdotme.wordpress.com/2011/09/25/many-ways-to-do-many-to-many-hacker-notes/

categorization.rb

class Categorization < ActiveRecord::Base
  attr_accessible :category_id, :product_id
  belongs_to :product
  belongs_to :category
end

category.rb

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

product.rb

class Product < ActiveRecord::Base
  attr_accessible :name
  has_many :categorizations
  has_many :categories, :through => :categorizations
end

product_new.html.erb

  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>
    <% @categories.each do |category| %>
        <%= category.name %>
    <%= check_box_tag "categories_names[]", category.name  %>

    <% end %>
  <div class="actions">
    <%= f.submit %>
  </div>

products_controller.rb

  # POST /products.json
  def create
    @product = Product.new(params[:product])


    if @product.save

      params[:categories_names].each do |item|
        add_category_tag = Category.where(:name => item)
        @product.categories << add_category_tag
      end

    end

rails generate migration create_categories_products_join

class CreateCategoriesProductsJoin < ActiveRecord::Migration
  def up
    create_table 'categories_products', :id => false do |t|
      t.column 'category_id', :integer
      t.column 'product_id', :integer
    end
  end

  def down
    drop_table 'categories_products'
  end
end