如何为包含删除时级联的ManyToMany关系创建Django迁移？

Question

我使用的是Django10、PostGres 3.9和Django3.2。我已经使用随附的多对多关系建立了这个模型...

class Account(models.Model):    
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    ...
    crypto_currencies = models.ManyToManyField(CryptoCurrency)

在生成并运行Django迁移之后，创建了下表...

\d cbapp_account_crypto_currencies;
                               Table "public.cbapp_account_crypto_currencies"
      Column       |  Type   |                                  Modifiers                                   
-------------------+---------+------------------------------------------------------------------------------
 id                | integer | not null default nextval('cbapp_account_crypto_currencies_id_seq'::regclass)
 account_id        | uuid    | not null
 cryptocurrency_id | uuid    | not null
Indexes:
    "cbapp_account_crypto_currencies_pkey" PRIMARY KEY, btree (id)
    "cbapp_account_crypto_cur_account_id_cryptocurrenc_38c41c43_uniq" UNIQUE CONSTRAINT, btree (account_id, cryptocurrency_id)
    "cbapp_account_crypto_currencies_account_id_611c9b45" btree (account_id)
    "cbapp_account_crypto_currencies_cryptocurrency_id_685fb811" btree (cryptocurrency_id)
Foreign-key constraints:
    "cbapp_account_crypto_account_id_611c9b45_fk_cbapp_acc" FOREIGN KEY (account_id) REFERENCES cbapp_account(id) DEFERRABLE INITIALLY DEFERRED
    "cbapp_account_crypto_cryptocurrency_id_685fb811_fk_cbapp_cry" FOREIGN KEY (cryptocurrency_id) REFERENCES cbapp_cryptocurrency(id) DEFERRABLE INITIALLY DEFERRED

我如何改变我的字段关系，或者生成一个迁移，使得级联关系是ON-DELETE级联？也就是说，当我删除一个帐户时，我希望该表中的伴随记录也被删除。

Answer 1

仔细看看这个。我尝试复制您的模型，并且我还发现中间表没有级联。我没有回答你关于如何添加级联的主要问题，但似乎django做了级联行为，这已经支持它：

当我删除一个帐户时，我希望同时删除此表中的附带记录。

要演示，请执行以下操作：

a = Account.objects.create(name='test')
c1 = CryptoCurrency.objects.create(name='c1')
c2 = CryptoCurrency.objects.create(name='c2')
c3 = CryptoCurrency.objects.create(name='c3')
a.crypto_currencies.set([c1, c2, c3])

如果您这样做了：

a.delete()

Django运行以下SQL，模拟中间表上的级联：

[
    {
        'sql': 'DELETE FROM "myapp_account_crypto_currencies" WHERE "myapp_account_crypto_currencies"."account_id" IN (3)', 'time': '0.002'
    }, 
    {
        'sql': 'DELETE FROM "myapp_account" WHERE "myapp_account"."id" IN (3)', 'time': '0.001'
    }
]

我在文档中找不到为什么它是这样做的。即使像这样添加自定义中介也会导致相同的行为：

class Account(models.Model):
    name = models.CharField(max_length=100)
    crypto_currencies = models.ManyToManyField(CryptoCurrency, through='myapp.AccountCryptocurrencies')


class AccountCryptocurrencies(models.Model):
    account = models.ForeignKey(Account, on_delete=models.CASCADE)
    cryptocurrency = models.ForeignKey(CryptoCurrency, on_delete=models.CASCADE)

Answer 2

当您使用ManyToManyField时，Django会为您创建一个中间表，在本例中名为cbapp_account_crypto_currencies。您将来要做的是始终显式地创建中间模型AccountCryptoCurrencies，然后设置ManyToManyField的through属性。这将允许您将来向中介模型中添加更多字段。更多信息请点击这里：https://docs.djangoproject.com/en/3.2/ref/models/fields/#django.db.models.ManyToManyField.through。

您现在需要做的就是创建这个中间表：

class AccountCryptoCurrencies(models.Model):
    account = models.ForeignKey(Account)
    cryptocurrency = models.ForeignKey(CryptoCurrency)

    class Meta:
        db_table = 'cbapp_account_crypto_currencies'

class Account(models.Model):    
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    ...
    crypto_currencies = models.ManyToManyField(CryptoCurrency, through=AccountCryptoCurrencies)

您现在需要生成一个迁移，但还没有应用它！通过将迁移包装在SeparateDatabaseAndState中来修改迁移。我还没有创建您的迁移文件，因为我没有完整的模型，但是您可以在这里查看如何创建迁移文件：How to add through option to existing ManyToManyField with migrations and data in django

现在可以应用迁移了，现在应该有了一个显式的中间表，而不会丢失数据。现在，您还可以向中间表添加其他字段并更改现有字段。您可以将on_delete=models.CASCADE添加到account字段并迁移更改。