private function setSingleValueRelationship($key, $foreign_entity)
{
if ($foreign_entity) {
$rel_metadata = $this->getMapper()->getEntityMetadata($foreign_entity);
$rel_reader = new Reader($rel_metadata, $foreign_entity);
$value = $rel_reader->getId();
} else {
$value = null;
}
$this->getDriver()->setSingleValueIndex($key, $value);
}