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_presence_of :category 
end

-OR-

class Post < ActiveRecord::Base
  belongs_to :category
  validates :category, presence: true
end

Rails versions prior to 3.2:

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