Reorganize registration_update async task, rename to section_demand_c…

…hange

backend/alert/models.py

@@ -487,7 +487,7 @@ def save(self, load_script=False, *args, **kwargs):
         and `current_demand_distribution_estimate` cache are asynchronously updated
         (via a celery task) to reflect the resulting section demand change.
         """
-        from alert.tasks import registration_update
+        from alert.tasks import section_demand_change
         from courses.util import get_set_id, is_fk_set
 
         # ^ imported here to avoid circular imports
@@ -534,9 +534,12 @@ def save(self, load_script=False, *args, **kwargs):
                 and self.section.semester == get_current_semester()
                 and was_active != self.is_active
             ):
-                registration_update.delay(
-                    self.section.id, was_active, self.is_active, self.updated_at
-                )
+                section = self.section
+                volume_change = int(self.is_active) - int(was_active)
+                if volume_change > 0 or section.registration_volume >= 1:
+                    section.registration_volume += volume_change
+                    section.save()
+                section_demand_change.delay(section.id, self.updated_at)
 
     def alert(self, forced=False, sent_by="", close_notification=False):
         """

backend/alert/tasks.py

@@ -71,28 +71,20 @@ def recompute_percent_open_async(semester):
 
 
 @shared_task(name="pca.tasks.registration_update")
-def registration_update(section_id, was_active, is_now_active, updated_at):
+def section_demand_change(section_id, updated_at):
     """
-    This method should only be called on sections from the current semester. It updates the
-    registration_volume of the registration's section, it updates the PcaDemandDistributionEstimate
-    models and current_demand_distribution_estimate cache to reflect the demand change.
+    This function should be called when a section's demand changes (i.e. the number of
+    active registrations changes, or the section's status is updated). It updates the
+    `PcaDemandDistributionEstimate` model and `current_demand_distribution_estimate`
+    cache to reflect the demand change.
+
+    :param: section_id: the id of the section involved in the demand change
+    :param: updated_at: the datetime at which the demand change occurred
     """
     section = Section.objects.get(id=section_id)
     semester = section.semester
-    assert (
-        semester == get_current_semester()
-    ), "Error: PCA registration from past semester cannot be updated."
-    if was_active == is_now_active:
-        # No change to registration volume
+    if semester != get_current_semester():
         return
-    volume_change = int(is_now_active) - int(was_active)
-    if volume_change < 0:
-        if section.registration_volume >= 1:
-            section.registration_volume += volume_change
-            section.save()
-    else:  # volume_change > 0
-        section.registration_volume += volume_change
-        section.save()
 
     with transaction.atomic():
         create_new_distribution_estimate = False

backend/courses/management/commands/merge_topics.py

@@ -47,7 +47,7 @@ def prompt_for_link(course1, course2):
 
 def same_course(course_a, course_b):
     return any(
-        course_bc.full_code == course_a.full_code
+        course_bc.full_code == course_a.primary_listing.full_code
         for course_bc in course_b.primary_listing.listing_set.all()
     )
 

backend/courses/models.py

@@ -7,7 +7,7 @@
 from django.core.exceptions import ValidationError
 from django.core.validators import MaxValueValidator, MinValueValidator
 from django.db import models, transaction
-from django.db.models import OuterRef, Q, Subquery
+from django.db.models import Case, OuterRef, Q, Subquery, Value, When
 from django.db.models.signals import post_save
 from django.dispatch import receiver
 from django.utils import timezone
@@ -326,26 +326,27 @@ def save(self, *args, **kwargs):
         if not self.topic:
             with transaction.atomic():
                 primary = self.primary_listing
-                topics = list(
-                    Topic.objects.select_related("most_recent")
-                    .filter(
-                        Q(most_recent__full_code=primary.full_code)
-                        | Q(most_recent__full_code__in=primary.listing_set.values("full_code")),
-                    )
-                    .select_related("most_recent")
-                )
-                if topics:
-                    topic = max(
-                        topics,
-                        key=lambda t: (
-                            int(t.most_recent.full_code == primary.full_code),
-                            t.most_recent.semester,
-                        ),
+                try:
+                    topic = (
+                        Topic.objects.filter(
+                            Q(most_recent__full_code=primary.full_code)
+                            | Q(most_recent__full_code__in=primary.listing_set.values("full_code")),
+                        )
+                        .annotate(
+                            most_recent_match=Case(
+                                When(most_recent__full_code=primary.full_code, then=Value(1)),
+                                default=Value(0),
+                                output_field=models.IntegerField(),
+                            )
+                        )
+                        .order_by("-most_recent_match", "-most_recent__semester")
+                        .select_related("most_recent")[:1]
+                        .get()
                     )
                     if topic.most_recent.semester < primary.semester:
                         topic.most_recent = primary
                         topic.save()
-                else:
+                except Topic.DoesNotExist:
                     topic = Topic(most_recent=primary)
                     topic.save()
 
@@ -818,6 +819,7 @@ def save(self, *args, **kwargs):
         it sets the percent_through_add_drop_period field.
         """
         from alert.models import validate_add_drop_semester
+        from alert.tasks import section_demand_change
         from courses.util import get_or_create_add_drop_period
 
         # ^ imported here to avoid circular imports
@@ -855,6 +857,8 @@ def save(self, *args, **kwargs):
         self.section.has_status_updates = True
         self.section.save()
 
+        section_demand_change.delay(self.section.id, self.created_at)
+
 
 """
 Schedule Models