errors with embedded docs after destroy_all

class Product
  include Mongoid::Document
  field :name

  embeds_many :product_items
  has_many_related  :articles

  def update_product_items
    product_items.destroy_all
    articles.all.each do |a|
      pi = product_items.new( a.attributes.reject{|k,v| k == "_id" || k == "_type" || k == "product_id" })
      pi.save
    end
  end
end