修复submission 列表展示非常慢的问题，不要去碰那些大字段

submission/migrations/0003_auto_20150821_1654.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('submission', '0002_submission_is_counted'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='submission',
+            name='problem_id',
+            field=models.IntegerField(db_index=True),
+        ),
+        migrations.AlterField(
+            model_name='submission',
+            name='user_id',
+            field=models.IntegerField(db_index=True),
+        ),
+    ]