Symfony 3.4表单中的CollectionType数组

I've got the array Cart of the entities, and I want to generate general form, which looks like on the screen.

As you see, I want to have editable field Quantity in each row, which represents the Cart entity, and I want to have ability to update all of them at once.

class Cart
{
/**
 * @ORM\Id
 * @ORM\Column(type="integer")
 * @ORM\GeneratedValue(strategy="AUTO")
 */
private $id;

/**
 * @ORM\ManyToOne(targetEntity="User", inversedBy="carts")
 */
private $userId;

/**
 * @ORM\ManyToOne(targetEntity="Product", inversedBy="carts")
 */
protected $product;

/**
 * @ORM\Column(type="integer")
 */
private $quantity;

/*gettes & setters */
}

For now, I have form, which wants to receive CollectionType, to work on it, but - I have only an array of entities, so It's dumping LogicalException.

What I need to do - there is any way to parse array to CollectionType, or maybe I could take group of cart entities from database in another way than that?:

$carts=$this->getDoctrine()->getRepository(Cart::class)->findByUserId($user);