how to send id_property by GET

[AmsTaFFix]

I have simple entity:

Class Book {
    private $id;
}
Class Page {
    private $book;
}

when i make get request, like localhost/pages, i have response like this

[
    23 => [
        'book'=>[
            'id'=>23
        ],
        'book_id'=>null
    ]
];

How i can get book_id, without create new field with same name in Page entity?