membershipworks: Switch EventExt.details_timestamp to an annotation

to avoid issues with saving new objects with GeneratedFields when the
pk is set

membershipworks/admin.py

@@ -146,7 +146,7 @@ class EventAdmin(DjangoObjectActions, admin.ModelAdmin):
             else:
                 fields.append(field.name)
         fields.insert(fields.index("end") + 1, "duration")
-        fields.append("details_timestamp")
+        fields.append("_details_timestamp")
         return fields
 
     @admin.display(ordering="duration")
@@ -160,6 +160,10 @@ class EventAdmin(DjangoObjectActions, admin.ModelAdmin):
             obj.url,
         )
 
+    @admin.display(description="Last details fetch")
+    def _details_timestamp(self, obj):
+        return naturaltime(obj.details_timestamp)
+
     @takes_instance_or_queryset
     def fetch_details(self, request, queryset):
         scrape_event_details(queryset)

membershipworks/models.py

@@ -511,6 +511,14 @@ class EventExtManager(models.Manager["EventExt"]):
                     * F("count"),
                     models.DurationField(),
                 ),
+                # TODO: this could be a GeneratedField, but that
+                # currently breaks saving when the primary key is
+                # provided (Django 5.0.1)
+                details_timestamp=Func(
+                    Func(F("details___ts"), function="FROM_UNIXTIME"),
+                    template="CONVERT_TZ(%(expressions)s, @@session.time_zone, 'UTC')",
+                    output_field=models.DateTimeField(),
+                ),
             )
         )
 
@@ -534,14 +542,6 @@ class EventExt(Event):
         max_digits=13, decimal_places=4, default=0
     )
     details = models.JSONField(null=True, blank=True)
-    details_timestamp = models.GeneratedField(
-        expression=Func(
-            Func(F("details___ts"), function="FROM_UNIXTIME"),
-            template="CONVERT_TZ(%(expressions)s, @@session.time_zone, 'UTC')",
-        ),
-        output_field=models.DateTimeField(),
-        db_persist=False,
-    )
 
     class Meta:
         verbose_name = "event"