class List
  include Mongoid::Document
  # Include just in case (probably already included)
  include ActiveModel::Serialization
  include ActiveModel::Serializers::JSON

  references_many :items
  field :name
end

class Item
  include Mongoid::Document
  field :name

  belongs_to :list
end

list = List.create(:name => 'list')
1.upto(2) {|i|
  list.items.create(:name => "item_#{i}")
}

puts "list.items.to_a.to_json=" + list.items.to_a.to_json #=> list.items.to_a.to_json=[{"_id":"4e51516eb4e4cf862a000002","list_id":"4e51516eb4e4cf862a000001","name":"item_1"},{"_id":"4e51516eb4e4cf862a000003","list_id":"4e51516eb4e4cf862a000001","name":"item_2"}]
puts "list.items.to_json=" + list.items.to_json #=> raises NoMethodError