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 method from links_controller.rb

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