Refactor part.on_order (#11496)

More intelligent DB querying for significant speed improvement

src/backend/InvenTree/part/models.py

@@ -2582,21 +2582,22 @@ class Part(
         Note that some supplier parts may have a different pack_quantity attribute,
         and this needs to be taken into account!
         """
+        from order.models import PurchaseOrderLineItem
+
         quantity = 0
 
-        # Iterate through all supplier parts
+        # Find all outstanding PurchaseOrderLineItem objects which reference this part
-        for sp in self.supplier_parts.all():
+        lines = PurchaseOrderLineItem.objects.filter(
-            # Look at any incomplete line item for open orders
-            lines = sp.purchase_order_line_items.filter(
             order__status__in=PurchaseOrderStatusGroups.OPEN,
+            part__part_id=self.pk,
             quantity__gt=F('received'),
-            )
+        ).prefetch_related('part')
 
         for line in lines:
             remaining = line.quantity - line.received
 
             if remaining > 0:
-                    quantity += sp.base_quantity(remaining)
+                quantity += line.part.base_quantity(remaining)
 
         return quantity