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I want to get primary key of a table in grid.php in my module.

$string_clik="'".str_replace("index.php/",'',Mage::getUrl()).'wedding_planner_invoice.php?&w_id='. $howToGetID."'";

How to get Id in grid?

protected function _prepareColumns()
        {
                $this->addColumn("wedding_planner_id", array(
                "header" => Mage::helper("planner")->__("ID"),
                "align" =>"right",
                "width" => "50px",
                "type" => "number",
                "index" => "wedding_planner_id",
                ));

                $this->addColumn("customer_name", array(
                "header" => Mage::helper("planner")->__("Name"),
                "index" => "customer_name",
                ));

                $this->addColumn("customer_email", array(
                "header" => Mage::helper("planner")->__("Email Address"),
                "index" => "customer_email",
                ));
                $this->addColumn('planner_date', array(
                        'header'    => Mage::helper('planner')->__('Order Date'),
                        'index'     => 'planner_date',
                        'type'      => 'datetime',
                    ));

                $string_clik="'".str_replace("index.php/",'',Mage::getUrl()).'wedding_planner_invoice.php?&w_id='. $this->wedding_planner_id."'";


                $this->addColumn('action',
                array(
                    'header'    => Mage::helper('planner')->__('Action'),
                    'width'     => '130px',
                    'type'      => 'action',
                    'getter'     => 'getId',
                    'actions'   => array(
                        array(
                            'caption' => Mage::helper('planner')->__('Generate Invoice'),
                            'url'     => 'javascript:void(0)',
                            'field'   => 'wedding_planner_id',
                            'onclick' => 'fnopenWindow('.$string_clik.')'
                        )
                    ),

                    'filter'    => false,
                    'sortable'  => false,
                    'index'     => 'stores',
                    'is_system' => true,
            ));
            $this->addExportType('*/*/exportCsv', Mage::helper('sales')->__('CSV')); 
            $this->addExportType('*/*/exportExcel', Mage::helper('sales')->__('Excel'));

                return parent::_prepareColumns();
        }