Warn about obsolete `update_alert_after` trigger.

README.md

@@ -52,11 +52,10 @@ in development environment.
         $ psql --user postgres
         postgres=# create database hc;
 
-* create database tables, triggers, superuser:
+* create database tables and the superuser account:
 
         $ cd ~/webapps/healthchecks
         $ ./manage.py migrate
-        $ ./manage.py ensuretriggers
         $ ./manage.py createsuperuser
 
 * run development server:

hc/api/management/commands/droptriggers.py

@@ -22,6 +22,7 @@ def _sqlite(cursor):
 
 class Command(BaseCommand):
     help = 'Drops the `update_alert_after` trigger'
+    requires_system_checks = False
 
     def handle(self, *args, **options):
         with connection.cursor() as cursor:

hc/api/models.py

@@ -7,6 +7,7 @@ from datetime import datetime, timedelta as td
 
 from croniter import croniter
 from django.conf import settings
+from django.core.checks import Warning
 from django.contrib.auth.models import User
 from django.db import models
 from django.urls import reverse
@@ -164,6 +165,29 @@ class Check(models.Model):
 
         return result
 
+    @classmethod
+    def check(cls, **kwargs):
+        errors = super(Check, cls).check(**kwargs)
+
+        trigger_detected = False
+        try:
+            dummy = Check(last_ping=timezone.now())
+            dummy.save()
+            dummy.refresh_from_db()
+            trigger_detected = bool(dummy.alert_after)
+            dummy.delete()
+        except:
+            pass
+
+        if trigger_detected:
+            err = Warning(
+                "Obsolete 'update_alert_after' trigger exists in database.",
+                hint="Please remove the trigger with 'manage.py droptriggers'",
+                id="hc.api.E001")
+            errors.append(err)
+
+        return errors
+
 
 class Ping(models.Model):
     n = models.IntegerField(null=True)