The magical document eating has_many assignment

Gemfile

source :rubygems
gem 'mongoid'

output.sh

mrkurt-2:mongoid-test kurt$ ./run.sh 

Parent has 10 kids
There are now 0 kids
mrkurt-2:mongoid-test kurt$ 

test.rb

require 'mongoid'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db("document_eater")
end

class Kid 
  include Mongoid::Document
  belongs_to :parent
end
class Parent
  include Mongoid::Document
  has_many :kids
end

Kid.destroy_all
Parent.destroy_all

p = Parent.create!
10.times do |n|
  k = Kid.create!
  p.kids << k
end

p.reload
puts "Parent has #{p.kids.count} kids"

kids = Kid.all.to_a
p = Parent.find(p.id)
p.kids = kids

puts "There are now #{Kid.count} kids"