undef keyword with symbol

I see something like this:

require "ostruct"
class OptionsWrapper < OpenStruct
  undef :id, :class

  def [](key)
    send(key)
  end

  def []=(key, value)
    send("#{key}=", value)
  end

  def method_missing(method, *args, &block)
    return @table.include?(method) ? @table.send(method) : nil if
%w(id class).include?(method.to_s)

    @table.respond_to?(method) ? @table.send(method, *args, &block) :
super
  end
end

I know OpenStruct is a class part of standard ruby library allowing
you to arbitrarily create methods on an object instance. I know that
undef allows you to set a method to nil. But I dont see why it is
needed in this case.

thanks for response

id and class are built-in ruby methods that are present on all object instances.

Rails uses them in a different manner than the default Ruby implementation.

They are undefined so that they can be picked up by the method_missing method.

that makes sense, thanks