Rails: Validating existence of an association

Question

I have a Category and a Post model, with each Post belonging to a Category. Before creating or updating a post, I need to check that the category selected exists. What's the best way to validate this information?

At the moment, I'm doing a find in the controller to ensure that the category exists. Is it possible to put these kinds of validations in the model?

Answer 1

http://blog.hasmanythrough.com/2007/7/14/validate-your-existence

class Post < ActiveRecord::Base
  belongs_to :category
  validates_existence_of :category 
end

Answer 2

I've put this in my model:

  validate :ensure_category_exists

  def ensure_category_exists
    errors.add('Category') unless self.blog.categories.find_by_id(self.category_id)
  end

Which prints "Category is invalid" if the category does not exist for the parent blog.