具有代理继承的 Django 模型多态性 [英] django model polymorphism with proxy inheritance

问题描述

My Discount 模型描述了系统中所有类型折扣的通用字段.我有一些代理模型，它们描述了计算总数的具体算法.基本的Discount 类有一个名为type 的成员字段，它是一个标识其类型及其相关类的字符串.

My Discount model describes common fields for all types of discounts in the system. I have some proxy models which describe concrete algorithm for culculating total. Base Discount class has a member field named type, which is a string identifing its type and its related class.

class Discount(models.Model):
  TYPE_CHOICES = (
    ('V', 'Value'),
    ('P', 'Percentage'),
  )

  name = models.CharField(max_length=32)
  code = models.CharField(max_length=32)
  quantity = models.PositiveIntegerField()
  value = models.DecimalField(max_digits=4, decimal_places=2)
  type = models.CharField(max_length=1, choices=TYPE_CHOICES)

  def __unicode__(self):
    return self.name

  def __init__(self, *args, **kwargs):
    if self.type:
      self.__class__ = getattr(sys.modules[__name__], self.type + 'Discount')
    super(Discount, self).__init__(*args, **kwargs)

class ValueDiscount(Discount):
  class Meta:
    proxy = True

  def total(self, total):
    return total - self.value

但我不断收到 AttributeError 异常，说 self 没有类型.如何解决这个问题，或者有其他方法可以实现吗?

But I keep getting exception of AttributeError saying self doesnt have type. How to fix this or is there another way to achieve this?

推荐答案

你的 init 方法需要看起来像这样:

Your init method needs to look like this instead:

def __init__(self, *args, **kwargs):
    super(Discount, self).__init__(*args, **kwargs)
    if self.type:
        self.__class__ = getattr(sys.modules[__name__], self.type + 'Discount')

在访问self.type之前，您需要调用super的__init__.

You need to call super's __init__ before you will be able to access self.type.

小心调用你的字段 type 因为 type 也是一个 python 内置函数，尽管你可能不会遇到任何问题.

Becareful with calling your field type since type is also a python built-in function, though you might not run into any problems.

参见:http://docs.python.org/library/functions.html#输入

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