Support Django 5.0 db_default attribute.

A DB default is not considered backward incompatible, as the default value is kept on DB-level.
See https://docs.djangoproject.com/en/dev/releases/5.0/#database-computed-default-values

tests/test_project/app_add_not_null_column_followed_by_db_default/migrations/0001_initial.py

@@ -0,0 +1,30 @@
+# Generated by Django 5.0.1 on 2024-02-04 20:07
+
+from __future__ import annotations
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = []
+
+    operations = [
+        migrations.CreateModel(
+            name="A",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("field", models.IntegerField()),
+            ],
+        ),
+    ]

tests/test_project/app_add_not_null_column_followed_by_db_default/migrations/0002_a_not_null_field_db_default.py

@@ -0,0 +1,21 @@
+# Generated by Django 5.0.1 on 2024-02-04 20:07
+
+from __future__ import annotations
+
+import django.db.models.functions.math
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("app_add_not_null_column_followed_by_db_default", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="a",
+            name="not_null_field_db_default",
+            field=models.IntegerField(db_default=django.db.models.functions.math.Pi()),
+        ),
+    ]

tests/test_project/app_add_not_null_column_followed_by_db_default/models.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+from django.db import models
+from django.db.models.functions import Pi
+
+
+class A(models.Model):
+    field = models.IntegerField()
+    not_null_field_db_default = models.IntegerField(
+        null=False,
+        db_default=Pi(),
+    )

tests/test_project/settings.py

@@ -14,6 +14,8 @@ from __future__ import annotations
 
 import os
 
+import django
+
 # Build paths inside the project like this: os.path.join(BASE_DIR, ...)
 BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 
@@ -59,6 +61,12 @@ INSTALLED_APPS = [
     "tests.test_project.app_create_index_exclusive",
 ]
 
+if django.VERSION[0] >= 5:
+    # db_default attribute was only added in Django 5.0
+    INSTALLED_APPS.append(
+        "tests.test_project.app_add_not_null_column_followed_by_db_default"
+    )
+
 MIDDLEWARE = [
     "django.middleware.security.SecurityMiddleware",
     "django.contrib.sessions.middleware.SessionMiddleware",