title: Rails: Validating existence of an association
body: <p>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?</p>
  
  <p>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?</p>