Make courses visible again

They were not visible, now they are

website/education/migrations/0002_auto_20161109_2024.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.3 on 2016-11-09 19:24
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('education', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='course',
+            name='until',
+            field=models.IntegerField(blank=True, null=True),
+        ),
+    ]

website/education/models.py

@@ -59,7 +59,7 @@ class Course(models.Model, metaclass=ModelTranslateMeta):
 
     since = models.IntegerField()
     until = models.IntegerField(
-        blank=True
+        blank=True, null=True
     )
 
     period = models.CharField(

website/education/views.py

@@ -14,7 +14,7 @@ from .models import Category, Course, Exam, Summary
 def courses(request):
     categories = Category.objects.all()
     objects = Course.objects.order_by('name_' + request.LANGUAGE_CODE).filter(
-        until=0)
+        until=None)
     return render(request, 'education/courses.html',
                   {'courses': objects, 'categories': categories})