fix: INC id increments have gaps

src/firetower/incidents/migrations/0008_gapless_incident_ids.py

@@ -0,0 +1,54 @@
+from django.db import migrations, models
+
+
+def seed_counter(apps, schema_editor):
+    """Seed the counter with MAX(id) + 1 or 2000 if no incidents exist."""
+    IncidentCounter = apps.get_model("incidents", "IncidentCounter")
+    Incident = apps.get_model("incidents", "Incident")
+
+    max_id = Incident.objects.aggregate(max_id=models.Max("id"))["max_id"]
+    next_id = (max_id + 1) if max_id is not None else 2000
+
+    IncidentCounter.objects.create(next_id=next_id)
+
+
+def reverse_seed_counter(apps, schema_editor):
+    """Remove the counter row."""
+    IncidentCounter = apps.get_model("incidents", "IncidentCounter")
+    IncidentCounter.objects.all().delete()
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("incidents", "0007_add_milestone_timestamps"),
+    ]
+
+    operations = [
+        # Change id field from AutoField to PositiveIntegerField
+        migrations.AlterField(
+            model_name="incident",
+            name="id",
+            field=models.PositiveIntegerField(primary_key=True, serialize=False),
+        ),
+        # Create the counter table
+        migrations.CreateModel(
+            name="IncidentCounter",
+            fields=[
+                (
+                    "id",
+                    models.BigAutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("next_id", models.PositiveIntegerField(default=2000)),
+            ],
+            options={
+                "db_table": "incidents_incident_counter",
+            },
+        ),
+        # Seed the counter with the appropriate starting value
+        migrations.RunPython(seed_counter, reverse_seed_counter),
+    ]

src/firetower/incidents/models.py

@@ -3,9 +3,30 @@
 from django.conf import settings
 from django.contrib.auth.models import AbstractUser, User
 from django.core.exceptions import ValidationError
-from django.db import models
+from django.db import models, transaction
 from django.db.models import Q, QuerySet
 
+INCIDENT_ID_START = 2000
+
+
+class IncidentCounter(models.Model):
+    """Stores the next available incident ID for gapless sequencing."""
+
+    next_id = models.PositiveIntegerField(default=INCIDENT_ID_START)
+
+    class Meta:
+        db_table = "incidents_incident_counter"
+
+
+def get_next_incident_id() -> int:
+    """Atomically get and increment the incident ID counter."""
+    with transaction.atomic():
+        counter = IncidentCounter.objects.select_for_update().get()
+        next_id = counter.next_id
+        counter.next_id += 1
+        counter.save()
+        return next_id
+
 
 class IncidentStatus(models.TextChoices):
     ACTIVE = "Active", "Active"
@@ -100,7 +121,8 @@ class Incident(models.Model):
     """
 
     # Primary key - numeric ID (2000, 2001, etc.)
-    id = models.AutoField(primary_key=True)
+    # Uses IncidentCounter for gapless sequential IDs
+    id = models.PositiveIntegerField(primary_key=True)
 
     # Core fields
     title = models.CharField(max_length=500)
@@ -235,9 +257,12 @@ def clean(self) -> None:
             raise ValidationError({"severity": "Severity must be set"})
 
     def save(self, *args: Any, **kwargs: Any) -> None:
-        """Override save to run validation"""
-        self.full_clean()
-        super().save(*args, **kwargs)
+        """Override save to run validation and assign gapless ID"""
+        with transaction.atomic():
+            if self._state.adding and self.id is None:
+                self.id = get_next_incident_id()
+            self.full_clean()
+            super().save(*args, **kwargs)
 
     def __str__(self) -> str:
         return f"{self.incident_number}: {self.title}"

src/firetower/incidents/tests/test_models.py

@@ -8,6 +8,7 @@
     ExternalLink,
     ExternalLinkType,
     Incident,
+    IncidentCounter,
     IncidentSeverity,
     IncidentStatus,
     Tag,
@@ -47,6 +48,35 @@ def test_incident_id_starts_at_2000(self):
         # Should be 2000 or higher (in case other tests ran first)
         assert incident.id >= 2000
 
+    def test_incident_ids_are_gapless_after_failed_save(self):
+        """Test that failed saves don't consume IDs (gapless sequence)"""
+        incident1 = Incident.objects.create(
+            title="First",
+            status=IncidentStatus.ACTIVE,
+            severity=IncidentSeverity.P1,
+        )
+        first_id = incident1.id
+
+        # Attempt to create an incident that will fail validation (empty title)
+        with pytest.raises(ValidationError):
+            Incident.objects.create(
+                title="",  # Invalid - will fail validation
+                status=IncidentStatus.ACTIVE,
+                severity=IncidentSeverity.P1,
+            )
+
+        # The counter should NOT have incremented due to the failed save
+        counter = IncidentCounter.objects.get()
+        assert counter.next_id == first_id + 1
+
+        # Create another valid incident - should be exactly first_id + 1
+        incident2 = Incident.objects.create(
+            title="Second",
+            status=IncidentStatus.ACTIVE,
+            severity=IncidentSeverity.P1,
+        )
+        assert incident2.id == first_id + 1
+
     def test_incident_number_property(self):
         """Test incident_number property returns correct format"""
         incident = Incident.objects.create(