python - Change class of child on django models

Question

Lets say I have classes in the form:

class A(models.Model):
   attrA = models.CharField()
class B(A):
   attrB = models.CharField()
class C(A):
   attrC = models.CharField()

And then I create a instnace of B:

b = B()

Now, based on some decisions I wanted to transform that object b an instance of the C class but with the attrC attribute available. Is that possible?

Answer 1

In python you can change class of an object like that:

b.__class__=C

Then all of B's attributes are available even when they are not defined for class C. Altough b is now instance of the class C it has no C's attributes. Before saving the object to database (or calling other methods of Model class) you have to add all remaining attributes of the class C. To prove it works I created a simple app. Here are my models:

class A(models.Model):
    attrA = models.CharField(max_length=128)
    class Meta:
        abstract=True
class B(A):
    attrB = models.CharField(max_length=128)
class C(A):
    attrC = models.CharField(max_length=128)

And here are my tests:

class ABCTestCase(TestCase):
    def test_changing_classes(self):
        """Changing classes"""
        a = A()
        self.assertIsInstance(a, A)
        a.attrA='bacon'
        self.assertEqual(a.attrA, 'bacon')
        a.__class__=B
        self.assertIsInstance(a, B)
        self.assertEqual(a.attrA, 'bacon')
        a.attrB='spam'
        self.assertEqual(a.attrA, 'bacon')
        self.assertEqual(a.attrB, 'spam')
        a.__class__=C
        self.assertIsInstance(a, C)
        self.assertIsInstance(a, A)
        self.assertNotIsInstance(a, B)
        a.attrC='egg'
        self.assertEqual(a.attrA, 'bacon')
        self.assertEqual(a.attrB, 'spam')
        self.assertEqual(a.attrC, 'egg')
        a.id=None
        a.save()
        self.assertIsNotNone(a.id)

Result of the test is OK.

Safer approach is to define method for each class which convert from B to C or from C to B.