polymorphic validation

Hello

I have 2 models.

link.rb

has_many :categories, :as => :categorized
validates_presence_of :name, :url, :created_at, :category

category.rb

belongs_to :categorized, :polymorphic => true
validates_presence_of :name

Everything seems to work. I select my category from a select tag. The
problem is i don't know how to validate if category is for example
empty
(nothing in a select tag), i would like to display the validation
error
along with other validation errors for link.

My create link controller.

def create
    @link = Link.new(params[:link])

    if @link.save
    flash[:notice] = 'Link was successfully created.'
    category = Category.new(params[:category][:id])
    category.categorized = @link
    category.save
    else
    render :action => "new"
    end
  end

First of all I am sorry for triple post (accident).
Thank you for reply. I will look into your solution.