django model polymorphism with proxy inheritance

Question

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

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

Derek Kwok · Accepted Answer · 2011-09-23 09:10:56Z

13

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')

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

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

See: http://docs.python.org/library/functions.html#type

answered Sep 23, 2011 at 9:10

Derek Kwok

13.1k7 gold badges41 silver badges58 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

aambrozkiewicz Over a year ago

Thank you. One more question, why do I need to call super init before accessing objects properties? I thought that it's a python which makes all members declared in object accessable in constructor, not super class (models.Model from Django). How come?

Derek Kwok Over a year ago

This is because django.db.models.Model has __metaclass__ = ModelBase. This means that django uses ModelBase to create your Model class instead of the regular type. I highly suggest reading: stackoverflow.com/questions/100003/… - and once you've mastered metaclasses, look at the django source code.

erikvw · Accepted Answer · 2011-09-23 09:11:14Z

1

call super(Discount, self).__init__(*args, **kwargs) before referencing self.type.

answered Sep 23, 2011 at 9:11

erikvw

4263 silver badges11 bronze badges

Collectives™ on Stack Overflow

django model polymorphism with proxy inheritance

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related