simplify using DATE_MAX as None/null by using a models.DateTimeField subclass

Author: jonatron

django_tasks/backends/database/apps.py

@@ -5,6 +5,3 @@ class TasksAppConfig(AppConfig):
     name = "django_tasks.backends.database"
     label = "django_tasks_database"
     verbose_name = "Tasks Database Backend"
-
-    def ready(self) -> None:
-        from . import signal_handlers  # noqa

django_tasks/backends/database/migrations/0017_alter_dbtaskresult_run_after.py

@@ -0,0 +1,19 @@
+# Generated by Django 5.1.4 on 2025-05-07 19:10
+
+import django_tasks.backends.database.models
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_tasks_database', '0016_alter_dbtaskresult_options_and_more'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='dbtaskresult',
+            name='run_after',
+            field=django_tasks.backends.database.models.NullIsMaxDateTimeField(verbose_name='run after'),
+        ),
+    ]

django_tasks/backends/database/models.py

@@ -50,6 +50,23 @@ def __class_getitem__(cls, _):
 DATE_MAX = datetime.datetime(9999, 1, 1, tzinfo=datetime.timezone.utc)
 
 
+class NullIsMaxDateTimeField(models.DateTimeField):
+    """
+    MySQL can only use indexes on simple queries, so instead of using null,
+    and nulls_last, or models.Case, set null/None as MAX_DATE, and vice-versa
+    """
+    def get_prep_value(self, value):
+        value = super().get_prep_value(value)
+        if value is None:
+            value = DATE_MAX
+        return value
+
+    def from_db_value(self, value, expression, connection):
+        if value == DATE_MAX:
+            return None
+        return value
+
+
 class DBTaskResultQuerySet(models.QuerySet):
     def ready(self) -> "DBTaskResultQuerySet":
         """
@@ -104,7 +121,7 @@ class DBTaskResult(GenericBase[P, T], models.Model):
     )
     backend_name = models.CharField(_("backend name"), max_length=32)
 
-    run_after = models.DateTimeField(_("run after"))
+    run_after = NullIsMaxDateTimeField(_("run after"))
 
     return_value = models.JSONField(_("return value"), default=None, null=True)
 
@@ -155,7 +172,7 @@ def task(self) -> Task[P, T]:
         return task.using(
             priority=self.priority,
             queue_name=self.queue_name,
-            run_after=None if self.run_after == DATE_MAX else self.run_after,
+            run_after=self.run_after,
             backend=self.backend_name,
         )