Fix annotated check-in list query

src/pretix/base/models/checkin.py

@@ -17,7 +17,18 @@ class CheckinList(LoggedModel):
 
     @staticmethod
     def annotate_with_numbers(qs, event):
-        from . import Order, OrderPosition
+        """
+        Modifies a queryset of checkin lists by annotating it with the number of order positions and
+        checkins associated with it.
+        """
+        # Import here to prevent circular import
+        from . import Order, OrderPosition, Item
+
+        # This is the mother of all subqueries. Sorry. I try to explain it, at least?
+        # First, we prepare a subquery that for every check-in that belongs to a paid-order
+        # position and to the list in question. Then, we check that it also belongs to the
+        # correct subevent (just to be sure) and aggregate over lists (so, over everything,
+        # since we filtered by lists).
         cqs = Checkin.objects.filter(
             position__order__event=event,
             position__order__status=Order.STATUS_PAID,
@@ -30,6 +41,11 @@ class CheckinList(LoggedModel):
         ).order_by().values('list').annotate(
             c=Count('*')
         ).values('c')
+
+        # Now for the hard part: getting all order positions that contribute to this list. This
+        # requires us to use TWO subqueries. The first one, pqs_all, will only be used for check-in
+        # lists that contain all the products of the event. This is the simpler one, it basically
+        # looks like the check-in counter above.
         pqs_all = OrderPosition.objects.filter(
             order__event=event,
             order__status=Order.STATUS_PAID,
@@ -41,10 +57,16 @@ class CheckinList(LoggedModel):
         ).order_by().values('order__event').annotate(
             c=Count('*')
         ).values('c')
+
+        # Now we need a subquery for the case of checkin lists that are limited to certain
+        # products. We cannot use OuterRef("limit_products") since that would do a cross-product
+        # with the products table and we'd get duplicate rows in the output with different annotations
+        # on them, which isn't useful at all. Therefore, we need to add a second layer of subqueries
+        # to retrieve all of those items and then check if the item_id is IN this subquery result.
         pqs_limited = OrderPosition.objects.filter(
             order__event=event,
             order__status=Order.STATUS_PAID,
-            item__in=OuterRef('limit_products')
+            item_id__in=Subquery(Item.objects.filter(checkinlist__pk=OuterRef(OuterRef('pk'))).values('pk'))
         ).filter(
             # This assumes that in an event with subevents, *all* positions have subevents
             # and *all* checkin lists have a subevent assigned
@@ -54,6 +76,9 @@ class CheckinList(LoggedModel):
             c=Count('*')
         ).values('c')
 
+        # Finally, we put all of this together. We force empty subquery aggregates to 0 by using Coalesce()
+        # and decide which subquery to use for this row. In the end, we compute an integer percentage in case
+        # we want to display a progress bar.
         return qs.annotate(
             checkin_count=Coalesce(Subquery(cqs, output_field=models.IntegerField()), 0),
             position_count=Coalesce(Case(