Magento Grid add new column - magento

Question

protected function _prepareCollection()
{
$collection = Mage::getResourceModel($this->_getCollectionClass());
$collection->getSelect()->join('sales_flat_order_address', 'main_table.entity_id = sales_flat_order_address.parent_id',array('company'));
$this->setCollection($collection);
}
I use the above code to add a company field on order listing grid.
but it shows "Item (Mage_Sales_Model_Order) with the same id "1038" already exist"

Answer 1

Maybe try an inner join instead:
$collection->getSelect()->joinInner(
array(
'order_address' => 'sales_flat_order_address'
),
'order_address.parent_id = main_table.entity_id'
);
Also, echo out your SQL and see what the collection returns, then try and run that sql on your database. This should help you figure out what you're doing wrong.
echo $collection->getSelect()->__toString();
Keep in mind that alone won't add the column to the grid. You'll need to add the column in _prepareColumns()
EDIT:
Actually, thinking about it, an inner join probably won't help you here. The issue you have is that sales_flat_order_address contains multiple entries for every parent_id, so you need to account for duplicates by using GROUP BY or SELECT DISTINCT. try something like this:
$collection->getSelect()->joinInner(
array(
'order_address' => 'sales_flat_order_address'
),
'order_address.parent_id = main_table.entity_id'
)->group(array('entity_id', 'parent_id'));
It's not perfect, but what you're trying to do is inherently imperfect, as one order has many addresses. The alternative is to explicitly join with only the billing address, or only the shipping address.