rails scope to check if association does NOT exist

I am looking toward writing a scope that returns all records that do not have a particular association.

foo.rb

class Foo < ActiveRecord::Base    
  has_many :bars
end

bar.rb

class Bar < ActiveRecord::Base    
  belongs_to :foo
end

I want a scope that can find all of the Foo's that dont have any bars. It's easy to find the ones that have an association using joins, but I haven't found a way to do the opposite.