public function add(\Tx_Oelib_Model $model)
    {
        if (!$model->hasUid()) {
            throw new \InvalidArgumentException('Add() requires a model that has a UID.', 1331488748);
        }

        $this->items[$model->getUid()] = $model;
        $this->highestUid = max($this->highestUid, $model->getUid());
    }