无法使用pre_save sign保存Django模型中的相关对象

Question

我必须在Django ORM中实现multi-aspect type of inheritance from UML。我有Contract数据类型，根据客户类型（普通客户或商业客户）可以分为RegularContract或{}。另外，合同可以有到期日或是不可到期的（它没有指定它的有效期），所以它也可以是ExpiringContract或{}类型。概念图如下：

我就是这样实现的：

模型.py代码：

class Contract(models.Model):
    approval_date = models.DateTimeField(null=False)

    def __getattr__(self, item):
        if self.expiringcontract:
            return getattr(self.expiringcontract, item)
        elif self.nonexpiringcontract:
            return getattr(self.nonexpiringcontract, item)


class ContractExpirationExtension(models.Model):
    base = models.OneToOneField("website.Contract",
                                on_delete=models.CASCADE)

    class Meta:
        abstract = True


class ExpiringContract(ContractExpirationExtension):
    termination_date = models.DateTimeField()

    @property
    def duration(self):
        return self.termination_date - self.base.approval_date


class NonExpiringContract(ContractExpirationExtension):

    @property
    def duration(self):
        return timedelta(days=100)


class ContractTypeExtension(models.Model):

    base = models.OneToOneField("website.Contract", on_delete=models.CASCADE)
    termination_delay = models.PositiveSmallIntegerField(default=30)

    class Meta:
        abstract = True

    @classmethod
    def create(cls, approval_date, contract_expiration_type, termination_delay, **kwargs):
        type_extension = cls(termination_delay=termination_delay)
        base = Contract(approval_date=approval_date)
        expiration_type = contract_expiration_type(**kwargs)
        expiration_type.base = base
        type_extension.base = base
        if contract_expiration_type.__name__ == ExpiringContract.__name__:
            type_extension.base.expiringcontract = expiration_type
        elif contract_expiration_type.__name__ == NonExpiringContract.__name__:
            type_extension.base.nonexpiringcontract = expiration_type
        return type_extension

    def __getattr__(self, item):
        if self.base:
            return getattr(self.base,item)


class RegularContract(ContractTypeExtension):
    termination_delay = models.PositiveSmallIntegerField(validators=[validate_term_delay_regular], blank=False)


class BusinessContract(ContractTypeExtension):
    termination_delay = models.PositiveSmallIntegerField(validators=[validate_term_delay_business], blank=False)

当我们需要创建新的契约模型实例时，我们从继承ContractTypeExtension抽象类的类中使用create()方法。在create()方法中，我基于传递给create()方法的类对象参数，创建Contract基本实例和相应的协定过期或非过期实例：

^{pr2}$

因为我的regular或business contract的实例中包含其他模型实例，所以我不能先保存base和expiration_type实例，因此我决定创建pre_savesignal，它将执行以下操作：

信号.py：

from django.db.models.signals import pre_save, pre_delete from django.dispatch import receiver

from .models import RegularContract, BusinessContract

@receiver(pre_save, sender=RegularContract) 
@receiver(pre_save, sender=BusinessContract) 
def pre_save_contract(sender, instance, *args,**kwargs):
    print("Pre_save")
    if not instance.id:
        instance.base.save()
        try:
            instance.base.expiringcontract.save()
        except (TypeError, ValueError):
            instance.base.nonexpiringcontract.save()

我在app的__init__和apps.py配置中注册了我的信号文件：

应用程序.py：

from django.apps import AppConfig


class WebsiteConfig(AppConfig):
    name = 'website'

    def ready(self):
       import website.signals

网站，网址：：

default_app_config = 'website.apps.WebsiteConfig'

为了测试我的代码，我编写了一些简单的测试用例：

class BusinessContractTestCase(TestCase):

    def setUp(self):
        pass

    def test_exprirating_creation(self):
        approval_date = datetime.today()
        termination_delay = 30
        termination_date = approval_date+timedelta(days=720)
        contract = BusinessContract.create(approval_date=approval_date,                                                         contract_expiration_type=ExpiringContract,
                                      termination_delay=termination_delay,
                                       termination_date=termination_date)
        contract.save()
        self.assertEqual(contract.termination_date.date(), ExpiringContract.objects.first().termination_date.date())


class RegularContractTestCase(TestCase):

    def test_exprirating_creation(self):
        approval_date = datetime.today()
        termination_delay = 30
        termination_date = approval_date + timedelta(days=720)
        contract = RegularContract.create(approval_date=approval_date,
                                      contract_expiration_type=ExpiringContract,
                                      termination_delay=termination_delay,
                                      termination_date=termination_date)
        contract.save()
        self.assertEqual(contract.termination_date.date(),
                           ExpiringContract.objects.first().termination_date.date())

但是当尝试运行这些测试时，它们失败了，我得到了这个错误：

Error
Traceback (most recent call last):
File "/home/ubuntu/workspace/webapp/website/tests.py", line 21, in test_exprirating_creation
contract.save()
File "/home/ubuntu/workspace/venv/lib/python3.5/site-packages/django/db/models/base.py", line 685, in save
"unsaved related object '%s'." % field.name
ValueError: save() prohibited to prevent data loss due to unsaved related object 'base'.

那么为什么我的代码中没有触发pre_save信号呢？在

Answer 1

经过短暂的调试后，我理解了我的问题（感谢Willem Van Onsem用pre_save指出了这个细节），这就是我解决问题的方法。我稍微修改了create()方法。我没有直接将base分配给新创建的实例，expiration_type给base，而是将这些保存到临时变量中，以后可以在signal方法中使用：

@classmethod
def create(cls, approval_date, contract_expiration_type, termination_delay, **kwargs):
    type_extension = cls(termination_delay=termination_delay)
    base = Contract(approval_date=approval_date)
    expiration_type = contract_expiration_type(**kwargs)
    type_extension.temp_base = base
    if contract_expiration_type.__name__ == ExpiringContract.__name__:
        type_extension.temp_expiringcontract = expiration_type
    elif contract_expiration_type.__name__ == NonExpiringContract.__name__:
        type_extension.base.temp_nonexpiringcontract = expiration_type
    return type_extension

然后在signals.pyin pre_save信号中，我将base与temporary variable分开保存，并分配给实例的base，并将我的到期/非expiring契约类型实例从临时变量分配到base并保存：

^{pr2}$

这可能不是最好的解决方案，但至少它是有效的。在