gistfile1.rb

require 'mongoid'

class Item; include Mongoid::Document; end

def clean_object(obj)
  obj.instance_variables.each do |ivar|
    obj.send(:remove_instance_variable, ivar) unless ivar == :@attributes
  end
end

item = Item.new
clean_object(Item.new)
item.instance_variables # => [:@attributes]