How to persist nested Doctrine Entities correctly - doctrine-orm

Question

I'm using Doctrine 2 in Zendframework 3.
I got an Order object with an 1:n relation to OrderAdditions which has a 1:1 Relation to OrderItems.
In the Order entity:
/**
* #ORM\OneToMany(targetEntity="LwsProject\Entity\OrderAddition", mappedBy="order", cascade={"persist", "remove"})
*/
protected $orderAdditions;
public function addOrderAdditions($orderAdditions) {
if($orderAdditions instanceof OrderAddition) {
$this->orderAdditions->add($orderAdditions);
$orderAdditions->setOrder($this);
}else if($orderAdditions instanceof ArrayCollection){
foreach($orderAdditions as $orderAddition){
$this->orderAdditions->add($orderAddition);
$orderAddition->setOrder($this);
}
}
return $this->orderAdditions;
}
public function removeOrderAdditions($orderAdditions) {
if($orderAdditions instanceof OrderAddition) {
$this->orderAdditions->removeElement($orderAdditions);
}else if($orderAdditions instanceof ArrayCollection){
foreach($orderAdditions as $orderAddition){
$this->orderAdditions->removeElement($orderAddition);
}
}
return $this->orderAdditions;
}
In the OrderAddition:
/**
* #ORM\OneToOne(targetEntity="\LwsProject\Entity\OrderItem", cascade={"persist", "remove"}, fetch="EAGER")
* #ORM\JoinColumn(name="order_item_id", referencedColumnName="id")
*/
protected $orderItem;
And in the OrderItem:
/**
* #ORM\OneToOne(targetEntity="LwsProject\Entity\OrderAddition")
* #ORM\JoinColumn(name="order_addition_id", referencedColumnName="id")
*/
protected $orderAddition;
Im Creating a new OrderAddition with a new OrderItem for several posted Items from a form.
// $order is either a new Order() or an entity from database.
$orderAddition = new OrderAddition();
//setting Data from post
$orderItem = new OrderItem();
//settign data from post
$orderAddition->setOrderItem($orderItem);
$orderItem->setOrderAddition($orderAddition);
$order->addOrderAdditions($orderAddition);
When I'm flushing while using $em->persist($order) with a new Order() everything gets persisted as expected, when im using an existing Order from the database i get:
SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'order_item_id' cannot be null