CakePHP 3.x Table::beforeMarshall

ParticipantsTable.php

<?php

class ParticipantsTable extends AppTable
{
    public function beforeMarshal(Event $event, \ArrayObject $data, \ArrayObject $options) {
      if(isset($data['partner']) && is_array($data['partner'])) {
        $participant = $this->get($data['id']);
        $data['partner']['participant_type_id'] = $participant->type_id;
      }
    }
}