Rename "session" field to "cart_id"

src/pretix/base/migrations/0004_auto_20151024_0848.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0003_event_is_public'),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name='cartposition',
+            old_name='session',
+            new_name='cart_id'
+        ),
+        migrations.AlterField(
+            model_name='cartposition',
+            name='cart_id',
+            field=models.CharField(blank=True, verbose_name='Cart ID (e.g. session key)', max_length=255, null=True),
+        ),
+    ]