SonataAdminBundle: Symfony 2.6.13 Sonata-Admin sonata_type_collection is breaking form

I’m using sonata_type_collection to create multiple inline photos for page:

$formMapper
->add('title', 'text', array('label' => 'Page title'))
->add('images','sonata_type_collection', array('label' => 'Images'), array(
   'edit' => 'inline',
   'inline' => 'table',
))

When I’m saving form I have the following error:

Extra fields are not allowed

Probably, because this is OneToMany relationship

/**
* @ORM\OneToMany(targetEntity="(..)\Image", mappedBy="page", cascade={"all"}, orphanRemoval=true)
*/
protected $images;

Doctrine doesn’t understand that something’s changed and therefore it doesn’t call prePersist/preUpdate.

public function preUpdate($product)
{
    die("Here"); // Doesn't die here.
    $product->setPhotos($product->getPhotos());
}

Maybe this is a reason? If so, how can I fix that?

When I’m trying to save form without any photos attached, it works flawlessly.