Doctrine 2 ManyToMany with Extra fields

example.php

<?php
// KIT
$product = new Product();
$product->setName('KIT 01')
        ->setPrice(9.99);

// SUB PRODUCT
$subProduct = new Product();
$subProduct->setName('SubProduct 02')
           ->setPrice(0.99);

// Persist first to get Id
$em->persist($product);
$em->persist($subProduct);
$em->flush();

// Relationship
$relation = new ProductRelation();
$relation->setAmount(5);

$product->addChildren($relation);
$subProduct->addParent($relation);


$em->flush();



// Finding a product with subproducts
$repository  = $em->getRepository('Entity\Product');

$product     = $repository->findWithChildrens(1);
$subProducts = $product->getChildrens();

printf("Product: %d - %s", $product->getId(), $product->getName());
echo "\nSub Products\n";
echo "total: ", $subProducts->count(), "\n\n";

foreach ( $subProducts as $r ) {
    echo "SubProduct\n";
    printf('Id: %s - Name: %s - Price: %f - Amount: %d',
        $r->getChildren()->getId(),
        $r->getChildren()->getName(),
        $r->getChildren()->getPrice(),
        $r->getAmount()
    );
    echo "\n";
}
echo "\n\n";

Product.php

<?php
namespace Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * Product entity
 *
 * @ORM\Table(name="catalog_product")
 * @ORM\Entity(repositoryClass="Repository\ProductRepository")
 */
class Product
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer", nullable=false)
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255, nullable=false)
     */
    private $name;

    /**
     * @var string
     *
     * @ORM\Column(name="price", type="decimal", precision=19, scale=2, nullable=false)
     */
    private $price;
    
    /**
     * @var Doctrine\Common\Collections\ArrayCollection|null
     * 
     * @ORM\OneToMany(
     *     targetEntity="ProductRelation",
     *     mappedBy="parent",
     *     cascade={ "persist", "remove" },
     *     orphanRemoval=TRUE,
     *     fetch="EXTRA_LAZY"
     * )
     * //@ORM\OrderBy({"name" = "ASC"})
     */
    private $parents;
    
    /**
     * @var Doctrine\Common\Collections\ArrayCollection
     * 
     * @ORM\OneToMany(
     *     targetEntity="ProductRelation",
     *     mappedBy="children",
     *     cascade={ "persist", "remove" },
     *     orphanRemoval=TRUE,
     *     fetch="EXTRA_LAZY"
     * )
     * //@ORM\OrderBy({"name" = "ASC"})
     */
    private $childrens;

    /**
     * Inicia um novo Produto
     * 
     * Popula a entidade com os dados em $data
     * 
     * @param array $data
     */
    public function __construct()
    {
        $this->parents   = new ArrayCollection();
        $this->childrens = new ArrayCollection();
    }

    /**
     * @return integer
     */
    public function getId()
    {
    	return $this->id;
    }
    
    /**
     * @param integer $id
     * @return Product
     */
    public function setId($id)
    {
    	$this->id = (int) $id;
    	return $this;
    }
    
    /**
     * @return string
     */
    public function getName()
    {
    	return $this->name;
    }
    
    /**
     * @param string $name
     * @return Product
     */
    public function setName($name)
    {
    	$this->name = (string) $name;
    	return $this;
    }
    
    public function setPrice($price)
    {
        $this->price = $price;
        
        return $this;
    }
    
    public function getPrice()
    {
        return $this->price;
    }
    
    public function getParents()
    {
        return $this->parents;
    }
    
    public function addParent(ProductRelation $relation)
    {
        $this->parents->add($relation);
        $relation->setChildren($this);
        
        return $this;
    }
    
    public function getChildrens()
    {
        return $this->childrens;
    }
    
    public function addChildren(ProductRelation $relation)
    {
        $this->childrens->add($relation);
        $relation->setParent($this);
        
        return $this;
    }
    
    public function addChildrens(array $childrens)
    {
        foreach ($childrens as $relation) {
            if (! $relation instanceof ProductRelation) {
                throw new \Exception('Subproduct should be instance of ProductRelation');
            }
            
            $this->addChildren($relation);
        }
        
        return $this;
    }
    
    public function toArray()
    {
        return array(
            'id'    => $this->getId(),
            'name'  => $this->getName(),
            'price' => $this->getPrice(),
        );
    }
}

ProductRelation.php

<?php
namespace Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
 * Product Relation entity
 *
 * @ORM\Table(name="catalog_product_relation")
 * @ORM\Entity
 */
class ProductRelation
{
    /**
     * @ORM\Id()
     * @ORM\ManyToOne(targetEntity="Product", inversedBy="parents")
     * @ORM\JoinColumn(name="parent_id", referencedColumnName="id", nullable=false)
     */
    protected $parent;

    /**
     * @ORM\Id()
     * @ORM\ManyToOne(targetEntity="Product", inversedBy="childrens")
     * @ORM\JoinColumn(name="child_id", referencedColumnName="id", nullable=false)
     */    
    protected $children;
    
    /**
     * @ORM\Column(name="amount", type="integer", nullable=false, options={"default": 1})
     */
    protected $amount;
    
    
    public function setParent(Product $parent = null)
    {
        $this->parent = $parent;
        
        return $this;
    }
    
    public function getParent()
    {
        return $this->parent;
    }
    
    public function setChildren(Product $children = null)
    {
        $this->children = $children;
        
        return $this;
    }
    
    public function getChildren()
    {
        return $this->children;
    }
    
    public function setAmount($amount)
    {
        $this->amount = (int) $amount;
        
        return $this;
    }
    
    public function getAmount()
    {
        return $this->amount;
    }
    
}

ProductRepository.php

<?php
namespace Repository;

use Entity\Product,
    Entity\ProductRelation;
    
use Doctrine\ORM\EntityRepository,
    Doctrine\ORM\Query\ResultSetMapping;

class ProductRepository extends EntityRepository
{
    public function findWithChildrens($id)
    {
        $product = parent::find($id);
        $this->fetchChildrens($product);
        
        return $product;
    }
    
    public function fetchChildrens(Product $product)
    {
        $sql = '
            SELECT
                pr.amount,
                p.id,
                p.name,
                p.price
            FROM
                catalog_product_relation pr
            INNER JOIN
                catalog_product p ON pr.child_id = p.id
            WHERE
                pr.parent_id = :parent_id
        ';
        
        $stmt = $this->_em->getConnection()->query($sql);
        $stmt->bindValue('parent_id', $product->getId());
        $stmt->execute();
        
        $result = $stmt->fetchAll();
        if (! empty($result)) {
            $subProducts = array();
            
            foreach ($result as $row) {
                $sub = new Product();
                $sub->setId($row['id'])
                    ->setName($row['name'])
                    ->setPrice($row['price']);
                
                $relation = new ProductRelation();
                $relation->setAmount($row['amount'])
                         ->setParent($product)
                         ->setChildren($sub);
                         
                $subProducts[] = $relation;
            }

            $product->addChildrens($subProducts);
            unset($sub, $relation, $subProducts);
        }
        
        return $product;
    }
}