Symfony 2.8 Doctrine. Using composite primary key and ManyToMany associations - symfony

Question

I have 2 entities - Platform and Product. In the Products table I have composite primary key by [Product ID + Platform ID]. One product can be present at many platforms so and one platform can contain many products, so the association is ManyToMany.
The Platform entity:
/**
* Platform
*
* #ORM\Table(name="Platforms")
*/
class Platform
{
/**
* #var int
*
* #ORM\Column(name="Platform_Id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #var string
*
* #ORM\Column(name="Platform_Name", type="string", length=64)
*/
private $name;
/**
* #ORM\ManyToMany(targetEntity="Product", mappedBy="platforms", cascade={"ALL"}, indexBy="numpp")
*/
protected $products;
public function __construct()
{
$this->products = new ArrayCollection();
}
public function addProducts($numpp)
{
$this->products[$numpp] = new Product($numpp, $this);
}
The Product entity:
/**
* Product
*
* #ORM\Table(name="Products")
*/
class Product
{
/**
* #var string
*
* #ORM\Column(name="Numpp", type="string", length=6)
* #ORM\Id
*/
private $numpp;
/**
* #var int
*
* #ORM\ManyToMany(targetEntity="Platform", inversedBy="products")
* #ORM\JoinColumn(name="Platform_Id", referencedColumnName="Platform_Id")
* #ORM\Id
*/
private $platforms;
public function __construct($numpp, Platform $platform)
{
$this->numpp = $numpp;
$this->platforms = new ArrayCollection();
$this->platforms[] = $platform;
}
In my controller when trying to create new Product entity...
$em = $this->getDoctrine()->getManager();
$platform = $em->getRepository("AGAAnalyticsBundle:Platform")->find(1);
$product = new Product('05062', $platform);
$em->persist($product);
$em->flush();
I get an error - Cannot insert the value NULL into column 'Platform_Id', table 'dbo.Products'
And other way using addProduct method...
$em = $this->getDoctrine()->getManager();
$platform = $em->getRepository("AGAAnalyticsBundle:Platform")->find(1);
$platform->addProduct('05062');
$em->flush();
I get an error - The column id must be mapped to a field in class AGA\AnalyticsBundle\Entity\Platform since it is referenced by a join column of another class.
Please help to understand where I am wrong and how I should build this relation between my entities correctly.