Missing metaclass in Django 1.5

问题

How do you override a model's metaclass in Django 1.5? I was overriding the metaclass on some models inheriting from an abstract model so I could set appropriate choices. e.g.

class BaseModel(models.Model):

    field_with_choices = models.CharField(max_length=100)

    class Meta:
        abstract = True

class MyModelMetaClass(BaseModel.__metaclass__):
    def __new__(cls, *args, **kwargs):
        new_class = super(MyModelMetaClass, cls).__new__(cls, *args, **kwargs)
        field = new_class._meta.get_field('field_with_choices')
        choices = field._choices = []
        choices.extend(get_choices())
        return new_class

class MyModel(BaseModel):

    __metaclass__ = MyModelMetaClass

However, when I upgraded to Django 1.5, I now get the error:

AttributeError: type object 'BaseModel' has no attribute '__metaclass__'

How do you override a model's metaclass in 1.5, or otherwise dynamically set field attributes in model subclasses?

回答1:

You can use the built-in function type like this:

class MyModelMetaClass(type(BaseModel)):
    def __new__(cls, *args, **kwargs):
        new_class = super(MyModelMetaClass, cls).__new__(cls, *args, **kwargs)
        field = new_class._meta.get_field('field_with_choices')
        choices = field._choices = []
        choices.extend(get_choices())
        return new_class

Although I see that type(ModelBase) is type so you basically could inherit from type or maybe from Model.__metaclass__ which is in turn ModelBase so ModelBase is the top of the Model metaclass architecture (before type of course :D ).

Hope it helps!

回答2:

If I understand correctly, django 1.5 is using six for compatibility management. According to the docs, the proper way would be the following:

from six import with_metaclass

class Meta(type):
    pass

class Base(object):
    pass

class MyClass(with_metaclass(Meta, Base)):
    pass

来源：https://stackoverflow.com/questions/16552812/missing-metaclass-in-django-1-5