Symfony2 detach entity with ManyToOne association - php

Question

Background:
In my application I have an entity that has a self referencing ManyToOne association (many children can point to a single parent). And I have a feature that does mass updates on many entities at one time using the Doctrine ORM. To keep performance from dropping dramatically due to many entities being loaded I detach entities once they've been updated.
Problem:
When I detach an entity that has children and later try to update any of those children Doctrine complains that it doesn't know the parent anymore. Even if I merge the parent entity before trying to update the child.
Question:
What am I doing wrong when I detach the parent entity? I've tried doing cascade="merge" and/or "detach" on the parent column and Doctrine still complains about the parent being an unknown entity when I try to persist.
I've mocked up a simple example that reproduces this. See below.
Test Code:
Entity\Thing.php
/**
* #ORM\Entity()
* #ORM\Table(name="things")
*/
class Thing
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #ORM\ManyToOne(targetEntity="Thing", inversedBy="children", cascade={"detach","merge"})
* #ORM\JoinColumn(name="parentId", referencedColumnName="id", onDelete="SET NULL")
*/
protected $parent;
/**
* #ORM\OneToMany(targetEntity="Thing", mappedBy="parent")
*/
protected $children;
/**
* #ORM\Column(type="string", length=64)
*/
protected $name;
public function __construct($name = null)
{
$this->children = new ArrayCollection();
$this->name = $name;
}
// .. SNIP ...
}
Test Action:
public function testThingAction($_route)
{
$em = $this->getDoctrine()->getEntityManager();
$repo = $em->getRepository('AcmeThingBundle:Thing');
// simple setup of a couple things in the DB
$t1 = $repo->findByName('Thing1');
if (!$t1) {
$t1 = new Thing('Thing1');
$t2 = new Thing('Thing2');
$t2->setParent($t1);
$em->persist($t1);
$em->persist($t2);
$em->flush();
return $this->redirect($this->generateUrl($_route));
}
list($t1, $t2) = $repo->findAll();
// detach and re-merge Thing1
// This should cause Thing1 to be removed and then re-added
// to the doctrine's known entities; but it doesn't!?
$em->detach($t1);
$em->merge($t1);
// try to update T2
$t2->setName('Thing2 - ' . time());
$em->persist($t2);
// will fail with:
// A new entity was found through the relationship Thing#parent
$em->flush();
return array();
}

Answer 1

The issue is that the child has a relationship to a specific parent object that is no longer managed by Doctrine. When you call $entityManager->merge($entity) you get a new managed entity back from that function.
When you get that back, you need to manually call setParent() on each of your children with the newly managed entity.