class Item
  include Mongoid::Document
  field name, type: String
end

class Pack
  include Mongoid::Document
end

class Pack
  include Mongoid::Document

  field items, type: Array
end

a = Pack.new
a.items = []
a.items << {item: Item.first, quantity: 4}
a.save

NoMethodError: undefined method `__bson_dump__' for #<Item:0x007faf1a56d670>

a = Pack.new
a.items = []
a.items << {item_id: Item.first.id, quantity: 4}
a.save

a.items[0].item.name

class Item
  include Mongoid::Document
  field name, type: String
  belongs_to :item_quantity
end

class ItemQuantity
  include Mongoid::Document
  has_one :item
  belongs_to :pack
  field quantity, type: Integer
end

class Pack
  include Mongoid::Document
  has_many :item_quantity
end

class Item
  include Mongoid::Document
  has_many :item_instances
end

class ItemInstance
  include Mongoid::Document
  belongs_to :item
  belongs_to :pack
end

class Pack
  include Mongoid::Document
  has_many :item_instances
end

pack = Pack.new
item = Item.first
pack.item_instances.build(item_id: item.id)

pack = Pack.new
product = Product.first
pack.items.build(product_id: product.id)

class Pack
  def new_item(product)
    self.items.build(product_id: product.id)
  end

  def products
    self.items.map { |i| i.product }
  end
end

pack.add_item(product)
# Same as using pack.items.build(product_id: product.id)

pack.products
# Returns an array (including duplicates) of all the products included in the pack.