CakePHP 3: How to update foreignKey?

I use BlueImp jQuery plugin to upload multiple images in follow scenario:

1. User opens add or edit Albums page, selects images and process upload via ajax method. This works fine, image data (name, size, model, field,..) are stored in related images table without foreign key.

2. Ajax return id of recent uploaded image, and js create (captions, position) inputs for that file.

3. User fills other form fields, and click submit.

Problem, if user creates a new album or edit/update existing one which does not contain images, application does not update foreign key for new images. But if album contains images, and user add new one to album, after save post CakePHP add / update foreign key to new images records.

Debug output:

// first time add images, does not update FK
[
    'name' => 'A4',
    'images' => [
        (int) 116 => [
            'caption' => '',
            'position' => '1',
            'id' => '116'
        ]
    ],
    'id' => '4',
    'user_id' => '1',
    'active' => '1'
]

// album has one image, we add new one, CakePHP Update FK for new images
[
    'name' => 'A4',
    'images' => [
        (int) 117 => [
            'caption' => '',
            'position' => '1',
            'id' => '117'
        ],
        (int) 118 => [
            'caption' => '',
            'position' => '2',
            'id' => '118'
        ]
    ],
    'id' => '4',
    'user_id' => '1',
    'active' => '1'
]

AlbumsController add method:

public function add()
{
    $album = $this->Albums->newEntity();
    if ($this->request->is('post')) {
        $album = $this->Albums->patchEntity($album, $this->request->data,['associated' => ['Images']]);
        if ($this->Albums->save($album)) {
            $this->Flash->success(__('The album has been saved.'));
            return $this->redirect(['action' => 'index']);
        } else {
            $this->Flash->error(__('The album could not be saved. Please, try again.'));
        }
    }
    $users = $this->Albums->Users->find('list', ['limit' => 200]);
    $this->set(compact('album', 'users'));
    $this->set('_serialize', ['album']);
}

Albums Table:

    $this->hasMany('Images', [
        'foreignKey' => 'foreign_key',
        'conditions' => [
            'Images.model' => 'Albums',
            'Images.field' => 'upload'
        ],
        'sort' => ['Images.position'  => 'ASC']
    ]);

Images Table:

    $this->belongsTo('Albums', [
        'foreignKey' => 'foreign_key',
        'joinType' => 'INNER'
    ]);

I uses the same approach in the CakePHP 2 apps, and worked without problems.

Video: http://screencast-o-matic.com/watch/cDhYYOinCX

Q: How to update foreignKey?

I'm not sure is this correct way, but it's work for me.

AlbumsTable

public function afterSave(Event $event, Entity $entity, ArrayObject $options)
{
   if (isset($entity)) {
        $images = TableRegistry::get('Images');
        $images->query()
            ->update()
            ->set(['foreign_key' => $entity->id])
            ->where([
                'foreign_key IS NULL',
                'model' => 'Albums'
                ])
            ->execute();// conditions
           }
}