ActiveRecord, Squeel: Getting objects without parent associations

Question

I have:

class ThingsCollection < ActiveRecord::Base
  has_and_belongs_to_many :things, uniq: true
end

class Thing < ActiveRecord::Base
  has_and_belongs_to_many :things_collections, uniq: true
end

I want to get Things which don't have any ThingsCollections (thing.things_colllection_ids.empty? == true). How can I do this via ActiveRecord & Squeel? (Without SQL query if possible)