定义徽章 只需将信号与sender = Badge连接，测试示例：
from django.db import models
from django.db.models import signals
def create_badge(sender, instance, created, **kwargs):
print "Post save emited for", instance
class BadgeCategory(models.Model):
name = models.CharField(max_length=100)
class Badge(models.Model):
name = models.CharField(max_length=16, help_text="Name for Badge")
category = models.ForeignKey(BadgeCategory, help_text="Category for badge")
description = models.CharField(max_length=32, help_text="A brief description")
signals.post_save.connect(create_badge, sender=Badge)
测试外壳会话：
In [1]: category = BadgeCategory(name='foo')
In [2]: category.save()
In [3]: badge = Badge(category=category, name='bar', description='test badge')
In [4]: badge.save()
Post save emited for Badge object
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