Symfony / doctrine insert related objects

Hoi,

ik heb een object "Order" en dat object heeft een array van orderDetails (classe: "OrderDetail").

Bij een nieuwe order wil ik zowel de order als de orderdetails in de database stoppen, en dat lukt perfect behalve de relatie tussen de twee objecten. Het "order_id" veld in de order_detail tabel blijft leeg. Hoe kan ik dat voorkomen?

Code waar ik de insert doe:

$order = $form->getData();
$this->om->persist($order);
$this->om->flush($order);

Dump van $order

object(Jds\ApiBundle\Entity\Order)[439]
  protected 'id' => null
  protected 'orderDate' => 
    object(DateTime)[611]
      public 'date' => string '2015-01-15 13:42:00' (length=19)
      public 'timezone_type' => int 3
      public 'timezone' => string 'Europe/Paris' (length=12)
  protected 'orderDetails' => 
    object(Doctrine\Common\Collections\ArrayCollection)[440]
      private '_elements' => 
        array (size=2)
          0 => 
            object(Jds\ApiBundle\Entity\OrderDetail)[667]
              protected 'id' => null
              protected 'product' => 
                object(Jds\ApiBundle\Entity\Product)[736]
                  ...
              protected 'price' => float 1
              protected 'amount' => int 2
              protected 'order' => null
          1 => 
            object(Jds\ApiBundle\Entity\OrderDetail)[702]
              protected 'id' => null
              protected 'product' => 
                object(Jds\ApiBundle\Entity\Product)[749]
                  ...
              protected 'price' => float 1
              protected 'amount' => int 5
              protected 'order' => null

Order-entity:
<?php
/**
* @ORM\Entity()
* @ORM\Table(name="the_order")
*/
class Order implements OrderInterface
{
/**
* @ORM\Id
* @ORM\Column(type="integer", length=6)
* @ORM\GeneratedValue(strategy="AUTO")
* @Serializer\Groups({"orderList"})
*/
protected $id;

/**
* @ORM\Column(type="datetime", nullable=false)
* @Serializer\Groups({"orderList"})
*/
protected $orderDate;

//protected $orderedBy;

//protected $handledBy;

/**
* @ORM\OneToMany(targetEntity="OrderDetail", mappedBy="order")
* @Serializer\Groups({"orderList"})
**/
protected $orderDetails;
[/code]

OrderDetail-entity
<?php
/**
* @ORM\Entity()
* @ORM\Table(name="order_detail")
*/
class OrderDetail
{
/**
* @ORM\Id
* @ORM\Column(type="integer", length=8)
* @ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;

/**
* @ORM\ManyToOne(targetEntity="Product")
* @Serializer\Groups({"orderList"})
*/
protected $product;

/**
* @ORM\Column(type="decimal", precision=4, scale=2)
*/
protected $price;

/**
* @ORM\Column(type="integer", length=2)
* @Serializer\Groups({"orderList"})
*/
protected $amount;

/**
* @ORM\ManyToOne(targetEntity="Order", inversedBy="orderDetails")
**/
protected $order;
[/code]