How to add an attribute to custom collection in admin grid?

Question

I have a 3rd party extension which provides a grid. Now I want to extend the grid with a simple Magento attribute. It looks so easy but I can't get it to work! I modified the function _prepareCollection() and wanted to add an attribute to the selection. Unfortunately all attempts failed.

That is my most promising solution (added before "setCollection") but it doesn't work either:

    $collection->getSelect()->join( array( '_atr'=>'eav_attribute'), "_atr.attribute_code = 'second_sku'", array());
    // $collection->addAttributeToSelect('second_sku');

Thats the (original) full function:

    protected function _prepareCollection() {
    $orderId = $this->getOrder()->getid();

    $collection = mage::getResourceModel('Purchase/OrderProduct_collection')
                    ->addFieldToFilter('pop_order_num', $orderId);

    //only products not supplied
    $collection->getSelect()->where('(pop_qty - pop_supplied_qty)  >0');

    //add image picture
    $smallImageTableName = mage::getModel('Purchase/Constant')->getTablePrefix() . 'catalog_product_entity_varchar';
    $collection->getSelect()->joinLeft($smallImageTableName,
            'pop_product_id=`' . $smallImageTableName . '`.entity_id and `' . $smallImageTableName . '`.store_id = 0 and `' . $smallImageTableName . '`.attribute_id = ' . mage::getModel('Purchase/Constant')->GetProductSmallImageAttributeId(),
            array('small_image' => 'value'));

    //join with product
    $productTableName = mage::getModel('Purchase/Constant')->getTablePrefix() . 'catalog_product_entity';
    $collection->getSelect()->joinLeft($productTableName,
            'pop_product_id=`' . $productTableName . '`.entity_id',
            array('sku' => 'sku'));

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

Any help will be much appreciated! Thank you!

EDIT: If I add my code to the original function I'll get following select:

SELECT `main_table`.*, `catalog_product_entity_varchar`.`value` AS `small_image`, `catalog_product_entity`.`sku` FROM `purchase_order_product` AS `main_table` LEFT JOIN `catalog_product_entity_varchar` ON pop_product_id=`catalog_product_entity_varchar`.entity_id and `catalog_product_entity_varchar`.store_id = 0 and `catalog_product_entity_varchar`.attribute_id = 109 LEFT JOIN `catalog_product_entity` ON pop_product_id=`catalog_product_entity`.entity_id INNER JOIN `eav_attribute` AS `_atr` ON _atr.attribute_code = 'second_sku' WHERE (pop_order_num = '1') AND ((pop_qty - pop_supplied_qty) >0);

As you see the select "works" but the attribute is not part of the selection. I already tried addAttributeToSelect and addFieldToSelect but both throw an error.

Answer 1

I finally got it done!

I added this in my _prepareCollection() function

        //ADD THE ATTRIBUTE 
    $this->_addSecondSku($collection);

Then I added a new function:

    protected function _addSecondSku($collection)
{
    $entityProduct = Mage::getSingleton("core/resource")->getTableName('catalog_product_entity');
    $entityValuesVarchar = Mage::getSingleton("core/resource")->getTableName('catalog_product_entity_varchar');
    $entityAtribute = Mage::getSingleton("core/resource")->getTableName('eav_attribute');

    $collection->getSelect()
        ->joinLeft( array( '_manAttr'=>$entityAtribute ), "_manAttr.attribute_code = 'second_sku'", array() )
        ->joinLeft( array( '_manValVarchar'=>$entityValuesVarchar ), "_manValVarchar.attribute_id = _manAttr.attribute_id AND _manValVarchar.entity_id = {$entityProduct}.entity_id", array('second_sku'=>'value'))
    ;
}

Answer 2

 protected function _prepareCollection()
    { 

        $collection = Mage::getResourceModel($this->_getCollectionClass());

        $collection->getSelect()->join('sales_flat_order', 'main_table.increment_id = sales_flat_order.increment_id',array('fee_amount'));

        $collection->join('invoice', 'main_table.entity_id = invoice.entity_id', 'subtotal');

        $this->setCollection($collection);

        return parent::_prepareCollection();
    }