Active Record: How to select records that have zero has_many relations?

Question

I'm using Rails 3, and I have the following:

class Artist < ActiveRecord::Base
  has_many :releases
  # more code
end

class Release < ActiveRecord::Base
  belongs_to :artist
  # more codez
end

If I want to select all artists that have NO releases, how would I do this without hardcoding SQL?