Make Answer.course_key non-nullable.

Previous migration populates any null course_key fields, so we can now
change the column to not allow NULL values.

We make the deprecated course_id nullable, to be able to start ignoring
it later.

problem_builder/migrations/0005_auto_20170112_1021.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem_builder', '0004_copy_course_ids'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='answer',
+            name='course_id',
+            field=models.CharField(default=None, max_length=50, null=True, db_index=True, blank=True),
+        ),
+        migrations.AlterField(
+            model_name='answer',
+            name='course_key',
+            field=models.CharField(max_length=255, db_index=True),
+        ),
+    ]

problem_builder/models.py

@@ -43,11 +43,9 @@ 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 is deprecated; it will be removed in next release.
-    course_id = models.CharField(max_length=50, db_index=True)
+    course_id = models.CharField(max_length=50, db_index=True, null=True, default=None)
     # course_key is the new course_id replacement with extended max_length.
-    # We need to allow NULL values during the transition period,
-    # but we will remove the null=True and default=None parameters in next release.
-    course_key = models.CharField(max_length=255, db_index=True, null=True, default=None)
+    course_key = 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)