How to test generated id in aggregate root

I wanna to test my entities created from doctrine, but i don`t know how to test id if they are generated themself while object save to database.

class Dish
{
    private ?int $id;

    private Collection $prices;

    public function __construct()
    {
        $this->prices = new ArrayCollection();
    }

    public function identifier(): string
    {
        return $this->id;
    }

    public function isEquals(Dish $dish): bool
    {
        return $this->identifier() === $dish->identifier();
    }

    public function addPrice(float $price)
    {
        $this->prices->add(new DishPrice($this, $price));
    }
}

class DishPrice
{
    use DeletedEntityTrait;

    private ?int $id;
    private float $price;

    private Dish $dish;

    public function __construct(Dish $dish, float $price)
    {
        $this->dish = $dish;
        $this->price = $price;
    }

    public function identifier(): string
    {
        return $this->id;
    }

    public function isEquals(Dish $dish): bool
    {
        return $this->identifier() === $dish->identifier();
    }
}

How to test isEquals methods in both classes?