Exclude FKed items from receipt discrepancies

This will let us add items to attendee receipts for purchases that don't affect their default cost without tripping the receipt discrepancies page.

The FK stands for Foreign Key.

uber/models/commerce.py

@@ -178,6 +178,12 @@ def item_total(self):
     def item_total_sql(cls):
         return coalesce(func.sum(ReceiptItem.amount * ReceiptItem.count), 0)
 
+    @classproperty
+    def fkless_item_total_sql(cls):
+        return coalesce(func.sum(ReceiptItem.amount * ReceiptItem.count).filter(
+            ReceiptItem.fk_id == None
+        ), 0)
+
     @property
     def txn_total(self):
         return self.payment_total - self.refund_total

uber/site_sections/reg_reports.py

@@ -46,9 +46,10 @@ def attendee_receipt_discrepancies(self, session, include_pending=False):
         else:
             filter = Attendee.is_valid == True  # noqa: E712
 
-        attendees = session.query(Attendee).filter(filter).join(  # noqa: E712
-                Attendee.active_receipt).outerjoin(ModelReceipt.receipt_items).group_by(
-                    ModelReceipt.id).group_by(Attendee.id).having(Attendee.default_cost_cents != ModelReceipt.item_total_sql)
+        attendees = session.query(Attendee).filter(
+            filter).join(Attendee.active_receipt).outerjoin(ModelReceipt.receipt_items).group_by(
+                ModelReceipt.id).group_by(Attendee.id).having(
+                    Attendee.default_cost_cents != ModelReceipt.fkless_item_total_sql)
 
         return {
             'attendees': attendees,