Using active record, how can I return the results for a model if they **do not** exist in a certain relationship elsewhere. For example, say I have a `Recipe` model which belongs to `categories` (via a `category_recipes` join table).

So basically I have a Rake task that looks through each recipe and it's description and try to put in in a category, but the task takes a long time and I'll need to run it on a regular basis, so I only want to run it if the `Recipe` in question has not already been categorized and I'd rather not add a superfluous column like `categorized`, so I figure there is a way to retrieve only the Recipes that don't exist in the join table. Any thoughts?