a.rb

require 'mongoid'

Mongoid.configure do |config|
  config.connect_to('test')
end

class Room
  include Mongoid::Document
  has_one :table, as: :parent, autobuild: true

  after_create :ensure_table

  def ensure_table
    table.save!
  end
end

class Table
  include Mongoid::Document

  belongs_to :parent, polymorphic: true
  embeds_many :chairs
end

class Chair
  include Mongoid::Document

  embedded_in :table
end


n = Table.count
m = 4

@room = Room.create!
@table = @room.table
@table.reload

m.times{ @table.chairs << Chair.new }

abort 'blargh' unless(Table.count == n + 1)
abort 'blurgh' unless(@table.chairs.size == m)
abort 'blurghy!' unless(Table.first.chairs.size == m)

puts Mongoid::VERSION #=> 3.1.6