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我升级到1.7.4的Django和使用新的内置的迁移，但我得到尝试运行makemigrations如果出现以下错误：的Python 2.7和Django 1.7迁移不受约束的方法
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：
class Discount(models.Model):
banner = models.ImageField(
help_text=_("Banner image for this discount"),
upload_to=upload_to('discounts/', 'title'),
blank=True,
null=True
)
而且我上传的回调app_utils.py：
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
根据错误消息，它表示upload_callback未被绑定。所以 我试图拉upload_callback以外的包装功能，但无法找到一种方法来传递path和attribute参数传递到upload_to。 Django的文档不清楚如何执行此操作，它仅指定需要instance和filename。
理想情况下，我想的是：
def upload_to(instance, filename, path, attribute=None):
...
任何想法如何，我可以去实现这一目标？