如何在Mongoid中更改文档的_type？

class NewClass< Mongoid::Document
  include Mongoid::Document
  store_in collection: 'old_collection_name'
  field :new_field, type: String
end

old_document = OldClass.where(id: 'old_document_id').first
new_document = NewClass.new(old_document.attributes.except('_type'))
new_document.save!
old_document.destroy

class OldClass
  include Mongoid::Document
  store_in collection: 'old_collection_name'
  field :old_field, type: String
  def self.inherited_class
    NewClass
  end
end