Django多表继承,如何知道哪个是模型的子类？

Question

我在django中遇到多表继承问题.

让我们以银行账户为例.

class account(models.Model):
    name = models……

class accounttypeA(account):
    balance = models.float…..

    def addToBalance(self, value):
        self.balance += value

class accounttypeB(account):
    balance = models.int…. # NOTE this

    def addToBalance(self, value):
        value = do_some_thing_with_value(value) # NOTE this
        self.balance += value

现在,我想为帐户类型添加一个值,但我拥有的只是一个帐户对象,例如acc = account.object.get(pk = 29).那么,谁是acc的孩子？

Django会自动在accounttypeA和accounttypeB中创建account_ptr_id字段.所以,我的解决方案是:

child_class_list = ['accounttypeA', 'accounttypeB']

for cl in child_class_list:
    try:
        exec(“child = ” + str(cl) + “.objects.select_for_update().get(account_ptr_id=” +              str(acc.id) + “)”)
        logger.debug(“Child found and ready to use.”)
        return child
    except ObjectDoesNotExist:
        logger.debug(“Object does not exist, moving on…”)

也许这是一个绘图板问题!:)

我希望在我的例子中我已经清楚了.谢谢

Answer 1

据我所知,没有Django内置的方法来做到这一点.

但是,给定acc=account.object.get(pk=29),您可以使用:

try:
    typeA = acc.accounttypeA
    # acc is typeA
except accounttypeA.DoesNotExist:
    # acc should be typeB if account only has typeA and typeB subclasses

try:
    typeB = acc.accounttypeB
    # acc is typeB
except accounttypeB.DoesNotExist:
    # acc should be typeA if account only has typeA and typeB subclasses

Answer 2

我的解决方案基于此

class account(models.Model):
    name = models……

    def cast(self):
        """
        This method is quite handy, it converts "self" into its correct child class. For example:

        .. code-block:: python

           class Fruit(models.Model):
               name = models.CharField()

           class Apple(Fruit):
               pass

           fruit = Fruit.objects.get(name='Granny Smith')
           apple = fruit.cast()

        :return self: A casted child class of self
        """
        for name in dir(self):
            try:
                attr = getattr(self, name)
                if isinstance(attr, self.__class__) and type(attr) != type(self):                 
                    return attr
            except:
                pass

    @staticmethod
    def allPossibleAccountTypes():
        #this returns a list of all the subclasses of account (i.e. accounttypeA, accounttypeB etc)
        return [str(subClass).split('.')[-1][:-2] for subClass in account.__subclasses__()]

    def accountType(self):
        try:
            if type(self.cast()) == NoneType:
                #it is a child
                return self.__class__.__name__
            else:
                #it is a parent, i.e. an account
                return str(type(self.cast())).split('.')[-1][:-2]
        except:
            logger.exception()
    accountType.short_description = "Account type"

class accounttypeA(account):
    balance = models.float…..

    def addToBalance(self, value):
        self.balance += value

class accounttypeB(account):
    balance = models.int…. # NOTE this