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Python Can'；t使用pre_save信号在django模型中保存相关对象

python django python-3.x

Python Can'；t使用pre_save信号在django模型中保存相关对象,python,django,python-3.x,Python,Django,Python 3.x,我必须用Django ORM实现。我有合同数据类型，根据客户类型（普通客户或业务客户）可分为常规合同或业务合同。此外，合同可以有到期日期，也可以不到期（未指定有效期），因此也可以是ExpiringContract或NonExpiringContract类型。这是概念图的外观： 我就是这样实现的： 型号.py代码： class Contract(models.Model): approval_date = models.DateTimeField(null=False) def

我必须用Django ORM实现。我有
合同
数据类型，根据客户类型（普通客户或业务客户）可分为
常规合同
或
业务合同
。此外，合同可以有到期日期，也可以不到期（未指定有效期），因此也可以是
ExpiringContract
或
NonExpiringContract
类型。这是概念图的外观：

我就是这样实现的：

型号.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，我们使用继承了
ContractTypeExtension
抽象类的类的
create（）
方法。在
create（）


@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


由于我的常规或业务合同实例中包含其他模型实例，因此如果不先保存
base
和
expiration\u type
实例，我无法保存它，因此我决定创建
pre\u save
信号，它将准确地执行以下操作：

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()



from django.apps import AppConfig


class WebsiteConfig(AppConfig):
    name = 'website'

    def ready(self):
       import website.signals



default_app_config = 'website.apps.WebsiteConfig'


我在app的
\uuuu init\uuuu
和
apps.py
config中注册了我的信号文件：

apps.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()



from django.apps import AppConfig


class WebsiteConfig(AppConfig):
    name = 'website'

    def ready(self):
       import website.signals



default_app_config = 'website.apps.WebsiteConfig'


网站。\uuuu init\uuuu.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()



from django.apps import AppConfig


class WebsiteConfig(AppConfig):
    name = 'website'

    def ready(self):
       import website.signals



default_app_config = 'website.apps.WebsiteConfig'


为了测试我的代码，我编写了简单的测试用例：


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())


但当尝试运行此测试时，它们失败了，我得到了以下错误：


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
 经过短暂的调试，我已经理解了我的问题（感谢您在
pre_save
中指出了这个细节），这就是我解决问题的方法。我稍微修改了
create（）
方法。我没有将
base
直接分配给新创建的实例，也没有将
expiration\u type
分配给base，而是将它们保存到临时变量中，以便以后在我的信号方法中使用：


@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.py
in
pre_save
signal中，我分别将base从临时变量保存到实例的base，并将其分配到实例的base，将我的到期/非到期合同类型实例从临时变量分配到base并保存：


@receiver(pre_save, sender=RegularContract)
@receiver(pre_save, sender=BusinessContract)
def pre_save_contract(sender, instance, *args, **kwargs):
    print("Pre_save")
    instance.temp_base.save()
    instance.base = instance.temp_base
    if hasattr(instance,"temp_expiringcontract"):
        instance.base.expiringcontract = instance.temp_expiringcontract
        instance.base.expiringcontract.save()
    else:
        instance.base.nonexpiringcontract =    instance.temp_nonexpiringcontract
        instance.base.nonexpiringcontract.save()



这可能不是最好的解决方案，但至少是可行的。
在
pre_save
级别，对象还没有主键，因此这就有点“鸡和蛋”的问题：您希望保存一个链接到尚未存储在数据库中的对象的对象。通常在这种情况下，最好执行
post\u save
。