Python 2.7与Django 1.7迁移中未绑定的方法问题

Question

我正在升级到Django 1.7.4并使用新的内置迁移，但是当我尝试运行makemigrations时出现以下错误：

ValueError: Could not find function upload_callback in app_utils.
Please note that due to Python 2 limitations, you cannot serialize unbound 
method functions (e.g. a method declared and used in the same class body). 
Please move the function into the main module body to use migrations.For more information, 
see https://docs.djangoproject.com/en/1.7/topics/migrations/#serializing-values

我的模型定义如下：

class Discount(models.Model):
    banner = models.ImageField(
        help_text=_("Banner image for this discount"),
        upload_to=upload_to('discounts/', 'title'),
        blank=True,
        null=True
    )

我在app_utils.py中的上传回调函数:

def upload_to(path, attribute=None):
    def upload_callback(instance, filename):
        compact = filename[:50]
        try:
            attr= unicode( slugify( getattr(instance, attribute) ) )
            mypath = '%s/%s/%s' % (path, attr, compact)
        except:
            mypath = '%s%s' % (path, compact)
        return mypath
    return upload_callback

根据错误信息，它表明upload_callback未绑定。因此，我尝试将upload_callback从包装函数中提取出来，但找不到一种方法来传递额外的path和attribute参数到upload_to中。Django的文档没有明确说明如何做到这一点，它只指定了需要instance和filename。

理想情况下，我希望是：

def upload_to(instance, filename, path, attribute=None):
    ...

有什么想法能帮我实现这个吗？

- Chris

Answer 1

在网上进行了一番深入的搜索后，我发现了这个Django的bug报告。解决方案似乎是创建一个可调用的工厂类：

https://code.djangoproject.com/ticket/22999

更多讨论请参见：Django - 无法为具有动态upload_to值的ImageField创建迁移

- Chris