cakephp3, raw query to update 2 tables

atable.php

<?php
  $this
    ->connection()
    ->newQuery()
    ->update('registrations, employees')
    ->set(['registrations.employee_id = employees.id'])
    ->where([
      'registrations.employee_id IS NULL',
      'registrations.employee_number = employees.employee_number',
    ])
    ->execute()
  ;