Requiring User to Select Categories -- (on Post Creation)


I'm having trouble with validation. I want to require the user to select a category before a post is successfully created.

I'm using has_many :through for posts and categories as follows:

class Post < ActiveRecord::Base   has_many :categorizations   has_many :categories, :through => :categorizations end

class Categorization < ActiveRecord::Base   belongs_to :category   belongs_to :post end

class Category < ActiveRecord::Base    has_many :categorizations    has_many :posts, :through => :categorizations end

I've tried validates_presence_of :categorizations in the Post model, but the application simply saves it without verifying that the user selected a checkbox.

Any ideas what I can do?