声明的顺序在models.py（Django / Python）中是否重要？ [英] Does order of declaration matter in models.py (Django / Python)?

问题描述

models.py

I have something like this in models.py

class ZipCode(models.Model):
    zip = models.CharField(max_length=20)
    cities = City.objects.filter(zip=self).distinct()

class City(models.Model):
    name = models.CharField(max_length=50)
    slug = models.CharField(max_length=50)
    state = models.ForeignKey(State)
    zip = models.ManyToManyField(ZipCode)

当我这样做时，我得到：

When I do this I get:

NameError: name 'City' is not defined

这是因为顺序声明有关系吗？如果是这样，我该怎么做，因为无论哪种方式安排它，看来我都会收到NameError。

Is this because the order of declaration matters? And if so, how can I do this, because either way I arrange this, it looks like I'm going to get a NameError.

谢谢。

推荐答案

是的，顺序确实很重要。

Yes order does matter as others have noted.

但是，遇到此问题几乎总是表明您做错了事。

Though, encountering this issue is almost always going to be an indication that you're doing something wrong.

在这种情况下，您的声明：

In this case your declaration:

cities = City.objects.filter(zip=self).distinct()

...都是多余和不好的做法。您可以通过在视图中引用该邮政编码的city_set来查找与该邮政编码相关的城市（即，不在您的模型中！）。因此，如果zip是ZipCode的实例，则可以执行以下操作：

... is both redundant and bad practice. You can find the cities related to a zip code by referring to that zip code's city_set in your views (ie not in your model!). So if zip is an instance of ZipCode, you would do:

cities = zip.city_set.all()

如果您真的想将其称为城市而不是 city_set，则可以在m2m声明中使用related_name参数。

If you really want to call it 'cities' rather than 'city_set' you can use the related_name parameter in your m2m declaration.

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