我很熟悉Django，但我最近注意到存在一个on_delete=models。级联选项与模型。我已经搜索了相同的文档，但我找不到更多的东西:
在Django 1.9中更改: On_delete现在可以用作第二个位置参数(以前它通常只作为关键字参数传递)。它将是Django 2.0中必须的参数。
用法示例如下:
from django.db import models
class Car(models.Model):
manufacturer = models.ForeignKey(
'Manufacturer',
on_delete=models.CASCADE,
)
# ...
class Manufacturer(models.Model):
# ...
pass
on_delete做什么?(我猜如果模型被删除了要做的动作。)
什么是模型。级联做什么?(文档中的提示)
还有什么其他的选择(如果我的猜测是正确的)?
这方面的文档放在哪里?
当前回答
简单地说，on_delete是一条指令，用于指定在删除外部对象时将对对象进行哪些修改:
CASCADE:当外部对象被删除时，将删除子对象
SET_NULL:将子对象外键设置为null
SET_DEFAULT:在创建模型时将子对象设置为给定的默认数据
RESTRICT:在某些条件下引发RestrictedError。
PROTECT:防止外部对象被删除，只要有子对象从它继承
额外的链接:
https://docs.djangoproject.com/en/4.0/ref/models/fields/#foreignkey
其他回答
使用CASCADE实际上意味着告诉Django删除引用的记录。 当一个“问题”被删除时，它也会删除这个问题的选项。
问:你是怎么知道我们公司的? (选择:1。朋友2。电视广告搜索引擎4。电子邮件推广)
当您删除这个问题时，它也会从表中删除所有这四个选项。 注意它流动的方向。 你不需要输入on_delete=models。问题级联模型把它放在选择中。
from django.db import models
class Question(models.Model):
question_text = models.CharField(max_length=200)
pub_date = models.dateTimeField('date_published')
class Choice(models.Model):
question = models.ForeignKey(Question, on_delete=models.CASCADE)
choice_text = models.CharField(max_legth=200)
votes = models.IntegerField(default=0)
删除父对象时删除数据库中的所有子字段，然后使用on_delete，如下所示:
class user(models.Model):
commodities = models.ForeignKey(commodity, on_delete=models.CASCADE)
简单地说，on_delete是一条指令，用于指定在删除外部对象时将对对象进行哪些修改:
CASCADE:当外部对象被删除时，将删除子对象
SET_NULL:将子对象外键设置为null
SET_DEFAULT:在创建模型时将子对象设置为给定的默认数据
RESTRICT:在某些条件下引发RestrictedError。
PROTECT:防止外部对象被删除，只要有子对象从它继承
额外的链接:
https://docs.djangoproject.com/en/4.0/ref/models/fields/#foreignkey
供您参考，模型中的on_delete参数与它听起来的相反。你把on_delete放在一个模型的外键(FK)上，告诉Django如果你在记录上指向的FK条目被删除了该怎么办。我们商店使用最多的选项是PROTECT、CASCADE和SET_NULL。以下是我总结出的基本规则:
Use PROTECT when your FK is pointing to a look-up table that really shouldn't be changing and that certainly should not cause your table to change. If anyone tries to delete an entry on that look-up table, PROTECT prevents them from deleting it if it is tied to any records. It also prevents Django from deleting your record just because it deleted an entry on a look-up table. This last part is critical. If someone were to delete the gender "Female" from my Gender table, I CERTAINLY would NOT want that to instantly delete any and all people I had in my Person table who had that gender. Use CASCADE when your FK is pointing to a "parent" record. So, if a Person can have many PersonEthnicity entries (he/she can be American Indian, Black, and White), and that Person is deleted, I really would want any "child" PersonEthnicity entries to be deleted. They are irrelevant without the Person. Use SET_NULL when you do want people to be allowed to delete an entry on a look-up table, but you still want to preserve your record. For example, if a Person can have a HighSchool, but it doesn't really matter to me if that high-school goes away on my look-up table, I would say on_delete=SET_NULL. This would leave my Person record out there; it just would just set the high-school FK on my Person to null. Obviously, you will have to allow null=True on that FK.
下面是一个模型的例子，它可以做到这三件事:
class PurchPurchaseAccount(models.Model):
id = models.AutoField(primary_key=True)
purchase = models.ForeignKey(PurchPurchase, null=True, db_column='purchase', blank=True, on_delete=models.CASCADE) # If "parent" rec gone, delete "child" rec!!!
paid_from_acct = models.ForeignKey(PurchPaidFromAcct, null=True, db_column='paid_from_acct', blank=True, on_delete=models.PROTECT) # Disallow lookup deletion & do not delete this rec.
_updated = models.DateTimeField()
_updatedby = models.ForeignKey(Person, null=True, db_column='_updatedby', blank=True, related_name='acctupdated_by', on_delete=models.SET_NULL) # Person records shouldn't be deleted, but if they are, preserve this PurchPurchaseAccount entry, and just set this person to null.
def __unicode__(self):
return str(self.paid_from_acct.display)
class Meta:
db_table = u'purch_purchase_account'
作为最后的花边新闻，您知道如果您没有指定on_delete(或没有指定)，默认行为是CASCADE吗?这意味着如果有人删除了您的gender表中的一个性别条目，那么具有该性别的任何Person记录也会被删除!
我会说，“如果有疑问，设置on_delete=models.PROTECT。”然后测试你的应用程序。您将很快发现哪些fk应该标记为其他值，而不会危及任何数据。
此外，值得注意的是，on_delete=CASCADE实际上没有添加到任何迁移中，如果这是您选择的行为的话。我想这是因为它是默认的，所以把on_delete=CASCADE等同于什么都不放。
CASCADE也会删除与之相连的对应字段。
