# app/models/shopping_cart.rb
class ShoppingCart
  include Mongoid::Document

  embeds_many :items
end

# app/models/item.rb
class Item
  include Mongoid::Document

  embedded_in :shopping_cart
end

2.5.3 :001 > cart = ShoppingCart.new
 => #<ShoppingCart _id: 5ce6a904a8f8432e27184e1b, > 
2.5.3 :002 > 32.times { cart.items.new }
 => 32 
2.5.3 :003 > cart.save!
 => true 
2.5.3 :004 > cart.items.map(&:destroy!).uniq
 => [true] 
2.5.3 :005 > cart.reload.items.count
 => 16 
2.5.3 :006 > cart.items.map(&:destroy!).uniq
 => [true] 
2.5.3 :007 > cart.reload.items.count
 => 8 
2.5.3 :008 > cart.items.all.map(&:destroy!).uniq
 => [true] 
2.5.3 :009 > cart.reload.items.count
 => 0