SerializationListener.php

    public function onPostSerialize(ObjectEvent $event)
    {
        $object = $event->getObject();
        $accessor = PropertyAccess::createPropertyAccessor();

        if($object instanceof ResourceInterface)
        {
            $metaData = $this->em->getClassMetadata($this->rm->getShortClass(get_class($object)));
            $associations = $metaData->associationMappings;

            foreach($associations as $name => $association)
            {
                $idField = $association['joinColumns'][0]['referencedColumnName'];
                $linkedAssociate = $accessor->getValue($object, $association['fieldName']);

                if(is_array($linkedAssociate))
                {
                    $ids = array();
                    foreach($linkedAssociate as $link)
                    {
                        $ids[] = $accessor->getValue($link, $idField);
                    }
                    $event->getVisitor()->addData($name, $ids);
                }
                else $event->getVisitor()->addData($name, $accessor->getValue($linkedAssociate, $idField));
            }
            //$event->getVisitor()->addData('someKey','someValue');
        }
    }