Closes #12135: Prevent the deletion of interfaces with children

docs/models/dcim/interface.md

@@ -77,6 +77,9 @@ If selected, this component will be treated as if a cable has been connected.
 
 Virtual interfaces can be bound to a physical parent interface. This is helpful for modeling virtual interfaces which employ encapsulation on a physical interface, such as an 802.1Q VLAN-tagged subinterface.
 
+!!! note
+    An interface with one or more child interfaces assigned cannot be deleted until all its child interfaces have been deleted or reassigned.
+
 ### Bridged Interface
 
 Interfaces can be bridged to other interfaces on a device in two manners: symmetric or grouped.

docs/models/virtualization/vminterface.md

@@ -16,6 +16,9 @@ The interface's name. Must be unique to the assigned VM.
 
 Identifies the parent interface of a subinterface (e.g. used to employ encapsulation).
 
+!!! note
+    An interface with one or more child interfaces assigned cannot be deleted until all its child interfaces have been deleted or reassigned.
+
 ### Bridged Interface
 
 An interface on the same VM with which this interface is bridged.

netbox/dcim/migrations/0182_protect_child_interfaces.py

@@ -0,0 +1,19 @@
+# Generated by Django 4.2.6 on 2023-10-20 11:48
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('dcim', '0181_rename_device_role_device_role'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='interface',
+            name='parent',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='child_interfaces', to='dcim.interface'),
+        ),
+    ]

netbox/dcim/models/device_components.py

@@ -537,7 +537,7 @@ class BaseInterface(models.Model):
     )
     parent = models.ForeignKey(
         to='self',
-        on_delete=models.SET_NULL,
+        on_delete=models.PROTECT,
         related_name='child_interfaces',
         null=True,
         blank=True,

netbox/virtualization/migrations/0037_protect_child_interfaces.py

@@ -0,0 +1,19 @@
+# Generated by Django 4.2.6 on 2023-10-20 11:48
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('virtualization', '0036_virtualmachine_config_template'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='vminterface',
+            name='parent',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='child_interfaces', to='virtualization.vminterface'),
+        ),
+    ]