Extend course_id field to 255 characters.

It stores course keys, which can be longer than 50 characters.

Standard field size for course key fields in edx-platform is 255
characters, so extend our course_id field to 255 chars.

problem_builder/migrations/0003_auto_20161116_0730.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem_builder', '0002_auto_20160121_1525'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='answer',
+            name='course_id',
+            field=models.CharField(max_length=255, db_index=True),
+        ),
+    ]

problem_builder/models.py

@@ -39,7 +39,7 @@ class Answer(models.Model):
 
     name = models.CharField(max_length=50, db_index=True)
     student_id = models.CharField(max_length=32, db_index=True)
-    course_id = models.CharField(max_length=50, db_index=True)
+    course_id = models.CharField(max_length=255, db_index=True)
     student_input = models.TextField(blank=True, default='')
     created_on = models.DateTimeField('created on', auto_now_add=True)
     modified_on = models.DateTimeField('modified on', auto_now=True)