public function groupByDataboxIdAndCollectionId()
    {
        $groups = [];

        foreach ($this->references as $index => $reference) {
            $databoxId = $reference->getDataboxId();
            $group = isset($groups[$databoxId]) ? $groups[$databoxId] : [];

            $group[$reference->getCollectionId()] = $index;
            $groups[$databoxId] = $group;
        }

        return $groups;
    }