Mongoid: order of referenced objects in many-to-many relation not as expected

gistfile1.rb

require 'mongoid'

puts "RUBY_VERSION: #{RUBY_VERSION}"
puts "Mongoid::VERSION: #{Mongoid::VERSION}"
puts

Mongoid.configure do |cfg|
  cfg.connect_to 'apples_and_oranges'
end



class Apple
  include Mongoid::Document
  has_and_belongs_to_many :oranges
end

class Orange
  include Mongoid::Document
  has_and_belongs_to_many :apples
end



a1, a2 = Apple.new, Apple.new

# behaviour changes when oranges are not persisted to db! (use .new instead of .create to reproduce)
o1, o2 = Orange.create!, Orange.create!


a1.oranges = [o1, o2]
p a1.orange_ids          # correct
p a1.oranges.map(&:id)   # correct

puts

a2.oranges = [o2, o1]
p a2.orange_ids          # correct
p a2.oranges.map(&:id)   # wrong