Fix for existing invalid cable types

Also hotfix against more in the future. Proper rework needed...This is why I should have waited for review...! Lesson learnt?

assets/migrations/0016_auto_20200413_1632.py

@@ -0,0 +1,34 @@
+# Generated by Django 3.0.3 on 2020-04-13 15:32
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('assets', '0015_remove_asset_next_sched_maint'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='cabletype',
+            name='circuits',
+            field=models.IntegerField(default=1),
+        ),
+        migrations.AlterField(
+            model_name='cabletype',
+            name='cores',
+            field=models.IntegerField(default=3),
+        ),
+        migrations.AlterField(
+            model_name='cabletype',
+            name='plug',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, related_name='plug', to='assets.Connector'),
+        ),
+        migrations.AlterField(
+            model_name='cabletype',
+            name='socket',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, related_name='socket', to='assets.Connector'),
+        ),
+    ]

assets/models.py

@@ -63,19 +63,23 @@ class Connector(models.Model):
         return self.description
 
 
+# Things are nullable that shouldn't be because I didn't properly fix the data structure when moving this to its own model...
 class CableType(models.Model):
     class Meta:
         ordering = ['plug', 'socket', '-circuits']
 
-    circuits = models.IntegerField(blank=True, null=True)
-    cores = models.IntegerField(blank=True, null=True)
-    plug = models.ForeignKey(Connector, on_delete=models.SET_NULL,
-                             related_name='plug', blank=True, null=True)
-    socket = models.ForeignKey(Connector, on_delete=models.SET_NULL,
-                               related_name='socket', blank=True, null=True)
+    circuits = models.IntegerField(default=1)
+    cores = models.IntegerField(default=3)
+    plug = models.ForeignKey(Connector, on_delete=models.CASCADE,
+                             related_name='plug', null=True)
+    socket = models.ForeignKey(Connector, on_delete=models.CASCADE,
+                               related_name='socket', null=True)
 
     def __str__(self):
-        return "%s → %s" % (self.plug.description, self.socket.description)
+        if self.plug and self.socket:
+            return "%s → %s" % (self.plug.description, self.socket.description)
+        else:
+            return "Unknown"
 
 
 @reversion.register