事情是这样的，我有一个存储考试的表
class Exam(models.Model):
category = cached_fields.ForeignKeyField(Category)
name = models.CharField(max_length=128)
date = models.DateField()
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
class Meta:
unique_together =
