private function selectByCategory($queryObject, $categoryID, $postTypeSlug)
    {
        $categoryRelationTable = categoriesRelationTable($postTypeSlug);
        return $queryObject->join($categoryRelationTable, $categoryRelationTable.'.postID', $postTypeSlug.'.postID')
            ->where($categoryRelationTable.'.categoryID', '=', $categoryID);
    }