Add management command to manually migrate course_id field.

Normally the value of deprecated Answer.course_id will be copied into
the new Answer.course_key field by a migration, but if your answer table
is very large, you might want to use this management command to perform
the copying in batches instead.

problem_builder/management/commands/copy_deprecated_course_id.py

+import math
+import time
+from django.core.management.base import BaseCommand
+from problem_builder.models import Answer
+
+
+class Command(BaseCommand):
+    """
+    Copy content of the deprecated Answer.course_id column into Answer.course_key in batches.
+    The batch size and sleep time between batches are configurable.
+    """
+    help = 'Copy content of the deprecated Answer.course_id column into Answer.course_key in batches'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--batch-size',
+            help='The size of each batch of records to copy (default: 5000).',
+            type=int,
+            default=5000,
+        )
+        parser.add_argument(
+            '--sleep',
+            help='Number of seconds to sleep before processing the next batch (default: 5).',
+            type=int,
+            default=5
+        )
+
+    def handle(self, *args, **options):
+        batch_size = options['batch_size']
+        sleep_time = options['sleep']
+        queryset = Answer.objects.filter(course_key__isnull=True)
+        batch_count = int(math.ceil(queryset.count() / float(batch_size)))
+
+        self.stdout.write(
+            "Copying Answer.course_id field into Answer.course_key in batches of {}".format(batch_size)
+        )
+
+        for batch in xrange(1, batch_count + 1):
+            for answer in queryset[:batch_size]:
+                answer.course_key = answer.course_id
+                answer.save()
+            self.stdout.write("Processed batch {} of {}".format(batch, batch_count))
+            if batch != batch_count:
+                time.sleep(sleep_time)
+
+        self.stdout.write("Successfully copied Answer.course_id into Answer.course_key")