How do I clear a collection in Doctrine? - doctrine-orm

Question

I have these two doctrine entity classes;
/** #ORM\Entity
* #ORM\Table(name="computer")
*/
class Computer extends Hardware
{
/**
* #ORM\OneToMany(targetEntity="LogicalDisk", mappedBy="computer", cascade={"persist", "remove"})
*/
protected $logicalDisks;
public function __construct()
{
$this->logicalDisks = new ArrayCollection();
}
public function addLogicalDisk($logicalDisk)
{
$this->logicalDisks[] = $logicalDisk;
$logicalDisk->setComputer($this);
}
public function clearLogicalDisks()
{
$this->logicalDisks->clear();
}
}
/** #ORM\Entity
* #ORM\Table(name="logical_disk")
*/
class LogicalDisk
{
/**
* #ORM\ManyToOne(targetEntity="Computer", inversedBy="logicalDisks")
* #ORM\JoinColumn(name="computer_id", referencedColumnName="id")
*/
protected $computer;
}
I'm trying to clear the existing collection, and then build up a new collection. However the old entities never get removed from the database.
$disks = $WbemServices->ExecQuery("Select * from Win32_LogicalDisk");
// Delete existing disk info for this host.
$computer = $service->findByDNSName($host, $domain);
$computer->clearLogicalDisks();
// loop through scanned disks and persist each one
foreach ($disks as $disk) {
$logicalDisk = new LogicalDisk();
$logicalDisk->setDeviceId($disk->DeviceId)
->setDescription($disk->Description)
->setFileSystem($disk->FileSystem)
->setCapacity($disk->Size)
->setFreeSpace($disk->FreeSpace);
$computer->addLogicalDisk($logicalDisk);
}
$service->persist($computer);
I have tried setting cascade={"persist", "remove"} on the logicalDisk property of the Computer entity, but this does not remove the items. I've also tried persisting the computer after clearing the ArrayCollection but the disk entities still do not get removed from the database.
I just need to know how to clear the ArrayCollection and then persist that change to the database, thus removing the unwanted entities.