public function groupBy($column)
    {
        $this->prepare();

        $data = [];
        foreach ($this->_map as $map) {
            $value = $this->getFromNested($map, $column);
            if ($value) {
                $data[$value][] = $map;
            }
        }

        $this->_map = $data;
        return $this;
    }