Django空字段回退 [英] Django empty field fallback

问题描述

我有一个保存用户地址的模型.此模型必须具有first_name和last_name字段，因为一个人想设置收件人的地址(例如他的公司等).我想要实现的是:

I have a model that holds user address. This model has to have first_name and last_name fields since one would like to set address to a recipient (like his company, etc.). What I'm trying to achieve is:

• 如果地址中的first_name/last_name字段已填写-请简单地返回该字段
• 如果地址中的first_name/last_name字段为空-从指向正确的django.auth.models.User
的外键中获取相应的字段数据
• 我希望将其视为将出现在字段查找中的普通Django字段
• 我不想创建一个方法，因为它是一种重构，并且Address.first_name/last_name在应用程序的各个位置(也包括模型形式等)中使用，所以我需要此方法尽可能地平滑，否则，我将不得不在很多地方进行修改.

• If the first_name/last_name field in the address is filled - return simply that field
• If the first_name/last_name field in the address is empty - fetch the corrresponding field data from a foreignkey pointing to a proper django.auth.models.User
• I'd like this to be treated as normal Django field that would be present in fields lookup
• I don't want to create a method, since it's a refactoring and Address.first_name/last_name are used in various places in the application (also in model forms, etc.), so I need this to me as smooth as possible, or else, I will have to tinker around in a lot of places.

推荐答案

此处有两个选项.第一种是创建一种方法来动态查找它，但是使用property装饰器，以便其他代码仍可以使用直接属性访问.

There are two options here. The first is to create a method to look it up dynamically, but use the property decorator so that other code can still use straight attribute access.

class MyModel(models.Model):
    _first_name = models.CharField(max_length=100, db_column='first_name')

    @property
    def first_name(self):
        return self._first_name or self.user.first_name

    @first_name.setter
    def first_name(self, value):
       self._first_name = value

这将始终引用first_name的最新值，即使相关的用户已更改.您可以像设置属性一样完全获取/设置属性:myinstance.first_name = 'daniel'

This will always refer to the latest value of first_name, even if the related User is changed. You can get/set the property exactly as you would an attribute: myinstance.first_name = 'daniel'

另一种选择是覆盖模型的save()方法，以便在保存时进行查找:

The other option is to override the model's save() method so that it does the lookup when you save:

def save(self, *args, **kwargs):
    if not self.first_name:
        self.first_name = self.user.first_name
    # now call the default save() method
    super(MyModel, self).save(*args, **kwargs)

通过这种方式，您不必更改数据库，但是仅在保存时刷新它-因此，如果相关的User对象被更改，但该对象没有更改，它将引用旧的User值.

This way you don't have to change your db, but it is only refreshed on save - so if the related User object is changed but this object isn't, it will refer to the old User value.

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