I have an `Author` model which habtm :feeds. Using Rails 3 want to setup a scope that finds all authors that have no associated feeds.

```
class Author < ActiveRecord::Base

    has_and_belongs_to_many :feeds
    scope :without_feed, joins(:feeds).where("authors_feeds.feed_id is null")

end

```

...doesn't seem to work. It feels like a simple thing. What am I missing here?