gistfile1.txt

<?= 
    GridInput::widget([
        'id' => 'detail-grid',
        'allModels' => $model->items,
        'model' => OrderItem::className(),
        'columns' => [
            ['class' => 'mdm\widgets\SerialColumn'],
            'product_id',
            'qty',
            [
                'attribute' => 'uom_id',
                'widget' => ['kartik\widgets\Select2', [
                    // option of select2
                ]]
            ],
            ['class' => 'mdm\widgets\ButtonColumn']
        ],
    ]);
?>