Fatal error on `prepareColumns` method

Question

Getting fatal error on Item (Mage_Catalog_Model_Product) with the same id "223" already exist

Here 223 is my product Id

My _prepareCollection() Method

protected function _prepareCollection()
{
    $coll = Mage::getModel('catalog/product')->getCollection()
                ->addAttributeToFilter('name',array('neq'=>''))  
                ->addAttributeToFilter('attribute_code',$this->getRequest()->getParam('my_value'));


    $coll->getSelect()->join( array('order_item'=> 'mgnt_sales_flat_order_item'), 'order_item.product_id = e.entity_id', array('order_item.created_at as ocreated','order_item.qty_ordered as sales_units', 'order_item.row_total as sales_total' , 'order_item.discount_amount as discount', 'order_item.item_id as item_id' ) );
    $coll->getSelect()->join( array('order'=> 'mgnt_sales_flat_order'), 'order.entity_id = order_item.order_id', array('customer_name'=>'order.customer_firstname'));



     $this->setCollection($coll);

    // echo $coll->getSelect(); 

     return parent::_prepareCollection();
}

My _prepareColumns() method

protected function _prepareColumns()
{ 


    $this->addColumn('entity_id', array(
        'header' => $this->_getHelper()->__('entity_id'),
        'type' => 'text',
        'index' => 'entity_id',
        'filter'    => false,
        'sortable'  => false
    ));
    $this->addColumn('name', array(
        'header' => $this->_getHelper()->__('Product Name'),
        'type' => 'text',
        'index' => 'name',
        'filter'    => false,
        'sortable'  => false
    ));

    $this->addColumn('customer_name', array(
        'header' => $this->_getHelper()->__('customer_name'),
        'type' => 'number',
        'index' => 'customer_name',
        'filter'    => false,
        'sortable'  => false
    ));
    $this->addColumn('sales_total', array(
        'header' => $this->_getHelper()->__('Sales Total'),
        'type' => 'number',
        'index' => 'sales_total',
        'filter'    => false,
        'sortable'  => false
    ));
    $this->addColumn('discount', array(
        'header' => $this->_getHelper()->__('Discount'),
        'type' => 'number',
        'index' => 'discount',
        'filter'    => false,
        'sortable'  => false
    ));



    return parent::_prepareColumns();

}

Answer 1

As per gopal's suggested link "You added something to the collection, which adds duplicates to the query result. What magento does with the query result is, to generate object from each row and then add these items to the collection. If the item already exists, this error is thrown."

As an hack you can try this for solution in your _prepareCollection() method:

$resource       = Mage::getModel('core/resource');
$connection     = $resource->getConnection('core_read');
$sql            = $coll->getSelect();
$rows           = $connection->fetchAll($sql);//this row will return an array

    $collection = new Varien_Data_Collection();
    foreach($rows as $row){
        $rowObj = new Varien_Object();
        $rowObj->setData($row);
        $collection->addItem($rowObj);
    }

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