# File lib/mongoid/document.rb, line 180
    def becomes(klass)
      unless klass.include?(Mongoid::Document)
        raise ArgumentError, "A class which includes Mongoid::Document is expected"
      end
      klass.instantiate(frozen? ? attributes.dup : attributes).tap do |became|
        became.instance_variable_set(:@errors, errors)
        became.instance_variable_set(:@new_record, new_record?)
        became.instance_variable_set(:@destroyed, destroyed?)
        became._type = klass.to_s
      end
    end