I want to use SS Shop to sell places at workshops. Its important that we don't oversell the workshops so I need to know how many places have been sold. Thats where I'm having problems.

The table OrderItem has the quantity,
Product_OrderItem extends OrderItem and contains the ProductID which identifies the workshop
Order contains the Status which tells me if the order has been paid
Order has a has-many relation with OrderItem
I'm sitting on the workshop page when I make the query so $this->ID gives me the ProductID.

Given all these elements, how do I construct a query that tells me how many of the current workshop 'products' exist in orders with a status of 'Paid'?

cheers
Bruce

I have been able to answer my own question for this one, courtesy of the very clear SilverStripe lessons. The following function gives me the number of sold items in paid invoices:

public function Paid() {
$StockTotal=0;
$StockItems = Product_OrderItem::get()->filter(array('ProductID' => $this->ID));
foreach ($StockItems as $Stockitem) {
	if ($Stockitem->Order()->Status=='Paid') {
	$StockTotal += $Stockitem->Quantity;
	}
}
return $StockTotal;
}