I am stuck with adding new column(Shipped date) to sales order admin grid. I copied the file
app/code/local/Mage/Adminhtml/Block/Sales/Order and in the
_prepareCollection() function added the code
$collection->getSelect()->joinLeft('sales_flat_shipment_grid','sales_flat_shipment_grid.order_id=main_table.entity_id',array('shipped_date' => 'sales_flat_shipment_grid.created_at'));
_prepareColumns() added this
$this->addColumn('shipped_date', array( 'header' => Mage::helper('sales')->__('Shipped Date'), 'index' => 'shipped_date', 'type' => 'datetime', 'filter_index'=>'sales_flat_shipment_grid.created_at', ));
My problem is when there are orders with multiple shipments and when I go to the sales order grid I get the error
There has been an error processing your request which says there is a duplicate order id.
If there are no multiple shipments for orders, then the sales order grid is fine and shows the shipped date for the corresponding orders.
Could someone help me with this?