How to pre-select a radio button in custom grid (M1)

Question

I'm having trouble figuring out how to load a custom grid with a radio button pre-selected.

For example, I added a custom "Tax Class" tab to the Admin Product Edit page, with a grid that loads the tax/class collection.

I already know the current product's assigned tax_class_id, Mage::registry('product')->getTaxClassId(), I just need to figure out how to select that corresponding row in the grid.

Any ideas are much appreciated, relevant code snippet below:

class Kmdsax_ProductTaxClassTab_Block_Adminhtml_Catalog_Product_Tab 
extends Mage_Adminhtml_Block_Widget_Grid
implements Mage_Adminhtml_Block_Widget_Tab_Interface {

public function __construct()
{
    parent::__construct();
    $this->setId('taxClassGrid');
    $this->setDefaultSort('class_name');        
    $this->setDefaultDir('ASC');
    $this->setSaveParametersInSession(true);
    $this->setUseAjax(true);
}

protected function _prepareCollection()
{
    $collection = Mage::getModel('tax/class')
        ->getCollection()                        
        ->setClassTypeFilter('PRODUCT');

    //$product = Mage::registry('product');
    //$taxClassId = $product->getTaxClassId();

    $this->setCollection($collection);
    return parent::_prepareCollection();
}

protected function _prepareColumns()
{
    $this->addColumn('tax_class_id', array(
        'header'    => Mage::helper('adminhtml')->__('Selected'),
        'type'      => 'radio',
        'html_name' => 'taxClassId',
        'align'     => 'center'
    ));

    $this->addColumn('op_avatax_code', array(
        'header'    => Mage::helper('tax')->__('Avatax Code'),
        'align'     => 'left',
        'index'     => 'op_avatax_code'
    ));

    $this->addColumn('class_name', array(
            'header'    => Mage::helper('tax')->__('Class Name'),
            'align'     => 'left',
            'index'     => 'class_name'
    ));

    return parent::_prepareColumns();
}

screenshot for context:

EDIT Thanks to Sohel Rana I got it working with the following modifications:

protected function _prepareColumns()
{
    $this->addColumn('tax_class_id', array(
        'header'    => Mage::helper('adminhtml')->__('Selected'),
        'type'      => 'radio',
        'html_name' => 'taxClassId',
        'align'     => 'center',
        'values'    => $this->_getSelectedTax(),
        'index'     => 'class_id'
    ));

    $this->addColumn('op_avatax_code', array(
        'header'    => Mage::helper('tax')->__('Avatax Code'),
        'align'     => 'left',
        'index'     => 'op_avatax_code'
    ));

    $this->addColumn('class_name', array(
            'header'    => Mage::helper('tax')->__('Class Name'),
            'align'     => 'left',
            'index'     => 'class_name'
    ));

    return parent::_prepareColumns();
}

protected function _addColumnFilterToCollection($column)
{
    // Set custom filter for selected tax-class
    if ($column->getId() == 'tax_class_id') {
        $taxClassIdArray = $this->_getSelectedTax();
        if (empty($taxClassIdArray)) {
            $taxClassIdArray = 0;
        }
        if ($column->getFilter()->getValue()) {
            $this->getCollection()->addFieldToFilter('class_id', array('in' => $taxClassIdArray));
        } else {
            if ($taxClassIdArray) {
                $this->getCollection()->addFieldToFilter('class_id', array('nin' => $taxClassIdArray));
            }
        }
    } else {
        parent::_addColumnFilterToCollection($column);
    }
    return $this;
}

protected function _getSelectedTax()
{
    $taxClassArray = array();
    $taxClassId = $this->getProduct()->getTaxClassId();

    $taxClassArray[0] = $taxClassId;
    return $taxClassArray;
}

protected function getProduct()
{
    $product = Mage::registry('product');
    if ($product == null) {
        $product = Mage::getModel('catalog/product')->load($this->getRequest()->getParam('id'));
    }
    return $product;
}

Answer 1

You should add values in following column

$this->addColumn('tax_class_id', array(
    'header'    => Mage::helper('adminhtml')->__('Selected'),
    'type'      => 'radio',
    'html_name' => 'taxClassId',
    'align'     => 'center',
    'values'            => $this->_getSelectedTax(),
));

Pass data to _getSelectedTax method.

Check app/code/core/Mage/Adminhtml/Block/Catalog/Product/Edit/Tab/Related.php as an example.