_category_fields.html.erb

<p>
  <%= f.label "Categories" %><br />
  <%= f.text_field :category_name, "data-pre" => @product.categories.map(&:attributes).to_json %>
</p>

Categorization.rb

class Categorization < ActiveRecord::Base
  attr_accessor :category_name

  belongs_to :product
  belongs_to :category

  def category_name
    category.try(:name)
  end  

  def category_name=(name)
    self.category = Category.find_or_create_by_name(name) if name.present?
  end
end

Product.rb

class Product < ActiveRecord::Base
  attr_accessible :name, :url, :popularity, :views, :description, :category_name 

  belongs_to :user
  has_many :categorizations
  has_many :categories, :through => :categorizations

#  accepts_nested_attributes_for :categories,
#                               :reject_if => lambda { |a| a[:content].blank? },
#                               :allow_destroy => true

## presence of
  validates :user_id, :presence => true
  validates :description, :presence => true, :length => {:minimum => 140 }

end

product_form.html.erb

  <div class="field">
    <%= f.fields_for :categorization do |build| %>
    <%= render "category_fields", :f => build %>
    <% end %>
  </div>
  

products_controller.rb

  def create
    @product = current_user.products.build(params[:product])

    respond_to do |format|
      if @product.save
        format.html { redirect_to root_url, notice: '#{current_user.handle}, your product was successfully added, now lets make it famous!.' }
        format.json { render json: @product, status: :created, location: @product }
      else
        format.html { render action: "new" }
        format.json { render json: @product.errors, status: :unprocessable_entity }
      end
    end
  end