class Car
  include Mongoid::Document
end

class Person
  include Mongoid::Document
  has_and_belongs_to_many :cars, inverse_of: nil
end

Car.delete_all
Person.delete_all

person = Person.create!

10.times do |i|
  person.cars << Car.create!
end

person = Person.includes( :cars ).first

# cars are not loaded 
# person.ivar( :cars ) === false