fix: restore has_timeslots order criteria for episode endpoint

The has_timeslots order criteria worked until commit
435e72d5 removed it from the default
queryset. This was the right choice in general because the annotation is
relatively expensive. However, it ignored the use in the ordering
filter.

To get the best of both worlds:
* fast query speed for queries that don’t care about timeslots
* filtering/ordering based on timeslots

this commit implements conditional ordering filter annotations.

refs dashboard#359

program/filters.py

@@ -55,6 +55,18 @@ class IntegerInFilter(filters.BaseInFilter):
         super().__init__(*args, **kwargs)
 
 
+class AnnotatedOrderingFilter(filters.OrderingFilter):
+    def __init__(self, *args, **kwargs):
+        self._filter_annotation_map = kwargs.pop("filter_annotation_map", {})
+        super().__init__(*args, **kwargs)
+
+    def filter(self, qs, value):
+        for key, annotation in self._filter_annotation_map.items():
+            if key in value or f"-{key}" in value:
+                qs = qs.annotate(**{key: annotation})
+        return super().filter(qs, value)
+
+
 class ShowOrderingFilter(filters.OrderingFilter):
     def filter(self, qs: QuerySet, value):
         if value in constants.EMPTY_VALUES:
@@ -289,9 +301,15 @@ class TimeSlotFilterSet(filters.FilterSet):
         ]
 
 
+episode_has_timeslots = Exists(models.TimeSlot.objects.filter(episode=OuterRef("pk")))
+
+
 class EpisodeFilterSet(StaticFilterHelpTextMixin, filters.FilterSet):
-    order = filters.OrderingFilter(
+    order = AnnotatedOrderingFilter(
         fields=["title", "id", "updated_at", "updated_by", "has_timeslots"],
+        filter_annotation_map={
+            "has_timeslots": episode_has_timeslots,
+        },
     )
     ids = IntegerInFilter(
         field_name="id",
@@ -310,7 +328,7 @@ class EpisodeFilterSet(StaticFilterHelpTextMixin, filters.FilterSet):
         help_text="Return only episodes that belong to the specified timeslot(s).",
     )
     has_timeslots = StaticQueryBooleanFilter(
-        query=Exists(models.TimeSlot.objects.filter(episode=OuterRef("pk"))),
+        query=episode_has_timeslots,
         label="Has timeslots",
         help_text="Returns only timeslots that either have or have not any timeslots.",
     )