Make user blankable

Fixes #38

pinax/referrals/migrations/0002_auto_20180130_0904.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.0.1 on 2018-01-30 09:04
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('referrals', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='referral',
+            name='user',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='referral_codes', to=settings.AUTH_USER_MODEL),
+        ),
+    ]

pinax/referrals/models.py

@@ -21,7 +21,8 @@ class Referral(models.Model):
         AUTH_USER_MODEL,
         on_delete=models.SET_NULL,
         related_name="referral_codes",
-        null=True
+        null=True,
+        blank=True
     )
     label = models.CharField(max_length=100, blank=True)
     code = models.CharField(max_length=40, unique=True)