Django multi-table inheritance, how to know which is the child class of a model?

Question

I'm having a problem with multi-table inheritance in django.

Let’s make an example with bank accounts.

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

Now, i want to add a value to an accounttype, but all i have is an account object, for instance acc=account.object.get(pk=29) . So, who is the child of acc ?

Django automatically creates an account_ptr_id field in accounttypeA and accounttypeB. So, my solution was:

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…”)

Maybe it's a drawing board problem at this point! :)

I hope I have been clear in my example. Thanks

K Z · Accepted Answer · 2012-10-06T01:57:48.700

11

To the best of my knowledge there isn't a Django built-in way to do this.

However, given acc=account.object.get(pk=29), you can use:

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

edited Oct 06 '12 at 01:57

answered Oct 06 '12 at 01:40

K Z

29,661
8
73
78

That was precisely what I did ;) You can see that above. – luistm Oct 06 '12 at 10:15

score 7 · Answer 2 · edited May 23 '17 at 10:30

my solution was based on this

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

Nikolai Saiko · Answer 3 · 2018-12-03T15:01:42.503

Django adds to class account two fields: accounttypea and accounttypeb. If you have accounttypeB object with pk=42, you can access from parent like this:

account.objects.get(pk=42).accounttypeb
>>> <accounttypeB instance>

You can add CharField to parent model with actual child-type for every child, and then use getattr, if there are a lot of child models (it may be better than a lot of try .. except xxx.DoesNotExist blocks).

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

    def ext(self):
        return getattr(self, self.cls.lower())


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

# example
accounttypeB.objects.create(balance=10,  name='Vincent Law', cls="accounttypeB")  
accounttypeA.objects.create(balance=9.5, name='Re-l Mayer', cls="accounttypeA")  
for obj in account.objects.all():
    obj.ext().addToBalance(1.0) 
    print(obj.name, obj.cls)

but you MUST create models using accounttypeA.objects.create(...) and accounttypeB.objects.create(...) - or else this trick will not work. (https://docs.djangoproject.com/en/1.5/topics/db/models/#multi-table-inheritance)

score 3 · Answer 4 · answered Nov 05 '19 at 08:24

You can use hasattr() method like:

if hasattr(account, 'accounttypea'):
   account.accounttypea.<somefield> = <some value>
   do something here....

elif hasattr(account, 'accounttypeb'):
   account.accounttypeb.<somefield> = <some value>
   do something here...

It's not so DRY but works. :)

Django multi-table inheritance, how to know which is the child class of a model?

4 Answers4

Linked