You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: principal_type.rb
    path of file: ./repos/openproject/lib/api/v3/principals
    
    the code of the file until where you have to start completion: #-- copyright
# OpenProject is an open source project management software.
# Copyright (C) 2012-2024 the OpenProject GmbH
#
# This program is free software; you can redistribute it and/or
# modify it under the terms
    
        def for(principal)
          case principal
          when User
            :user
          when Group
            :group
          when PlaceholderUser
            :placeholder_user
          when NilClass
            # Fall back to user for unknown principal
            # since we do not have a principal route.
            :user
          else
            raise "undefined subclass for #{principal}"
          end
        end