children/child annotation add value later

when using the child annotation, it is not possible to assign a child document with a child of its own after persisting the parent document. to reproduce, take the Functional/FileTest.php and change
$parent->file = new File();
$parent->id = '/functional/filetest';
$parent->file->setFileContentFromFilesystem(dirname(_FILE_) .'/_files/foo.txt');

$this->dm->persist($parent);

to
$parent->id = '/functional/filetest';
$this->dm->persist($parent);

$parent->file = new File();
$parent->file->setFileContentFromFilesystem(dirname(_FILE_) .'/_files/foo.txt');

you will get "mandatory child node

content does not exist"