public function getSubset($prop, $keys) {
		if (is_string($keys))
			$keys = \Base::instance()->split($keys);
		if (!is_array($keys))
			trigger_error(sprintf(self::E_SubsetKeysValue,gettype($keys)),E_USER_ERROR);
		if (!$this->hasRelSet($prop) || !($relSet = $this->getRelSet($prop)))
			return null;
		foreach ($keys as &$key) {
			if ($key instanceof \MongoId || $key instanceof \MongoDB\BSON\ObjectId)
				$key = (string) $key;
			unset($key);
		}
		return array_values(array_intersect_key($relSet, array_flip($keys)));
	}