删除 ManyToMany 字段无效的 null=True

problem/migrations/0004_auto_20150812_2254.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('problem', '0003_auto_20150810_2233'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='problem',
+            name='tags',
+            field=models.ManyToManyField(to='problem.ProblemTag'),
+        ),
+    ]

problem/models.py

@@ -49,4 +49,4 @@ class Problem(AbstractProblem):
     # 难度 0 - n
     difficulty = models.IntegerField()
     # 标签
-    tags = models.ManyToManyField(ProblemTag, null=True)
+    tags = models.ManyToManyField(ProblemTag)