/**
 * @ORM\Entity
 */
class Category
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue
     */
    private $id;
    /**
     * @ORM\OneToMany(targetEntity="Product", mappedBy="category", fetch="EAGER")
     */
    private $products;

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

    public function products(): Collection
    {
        return $this->products;
    }

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

    public function addProduct(Product $product)
    {
        $product->setCategory($this);
        $this->products->add($product);
    }
}

/**
 * @ORM\Entity
 */
class Product
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue
     */
    protected $id;

    /**
     * @ORM\ManyToOne(targetEntity="Category", inversedBy="products")
     * @ORM\JoinColumn(name="category_id", referencedColumnName="id")
     */
    private $category;

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

    public function setCategory(Category $category)
    {
        $this->category = $category;
    }

    public function category()
    {
        return $this->category;
    }
}


$cat = new Category();
$cat->addProduct(new Product());

$this->entityManager->persist($prod);
$this->entityManager->persist($cat);
$this->entityManager->flush();