support display id

problem/migrations/0002_problem__id.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.12 on 2017-02-09 08:45
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='problem',
+            name='_id',
+            field=models.CharField(db_index=True, default='1', max_length=24, unique=True),
+            preserve_default=False,
+        ),
+    ]