client.0.id, client.0.name ...
client.1.id, client.1.name ...

srcControllerBillsController.php (line 145)
    [
        'activity' => 'shopping',
        'bill_date' => '24-01-2018',
        'client' => [
            (int) 0 => [
                'nif' => '12341234D',
                '_joinTable' => [
                    'amount' => '100'
                ],
                'id' => '2'
            ],
            (int) 1 => [
                'nif' => '12345678Z',
                '_joinTable' => [
                    'amount' => '120'
                ],
                'id' => '3'
            ]
        ],
        'event' => [
...