Should be one-to-one relationship.

bikeshop_project/core/migrations/0013_auto_20160531_0236.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.4 on 2016-05-31 02:36
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0012_auto_20160531_0234'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='payment',
+            name='membership',
+        ),
+        migrations.AddField(
+            model_name='membership',
+            name='payment',
+            field=models.OneToOneField(null=True, on_delete=django.db.models.deletion.CASCADE, related_name='membership', to='core.Payment'),
+        ),
+    ]

bikeshop_project/core/models.py

@@ -16,6 +16,13 @@ class Membership(models.Model):
         blank=True,
         null=True
     )
+    payment = models.OneToOneField(
+        'Payment',
+        on_delete=models.CASCADE,
+        related_name='membership',
+        blank=False,
+        null=True
+    )
 
 
 class Payment(models.Model):
@@ -28,13 +35,6 @@ class Payment(models.Model):
     )
     type = models.CharField(max_length=12, choices=payment_choices)
     created_at = models.DateTimeField(auto_now_add=True)
-    membership = models.ForeignKey(
-        'Membership',
-        on_delete=models.CASCADE,
-        related_name='payments',
-        blank=False,
-        null=True
-    )
 
 
 class Visit(models.Model):