Question

Can someone explain this simple, yet deceiving, anomaly?

There are two models, where B is a sub-model of A:

# models.py
class A(models.Model):
    a = models.IntegerField(blank=True)

class B(A):
    b = models.IntegerField(blank=True)

Simple, right? Yet in runtime:

>>> A()
<A: A object>
>>> B()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "django/db/models/base.py", line 357, in __init__
    setattr(self, field.attname, val)
  File "django/db/models/fields/related.py", line 271, in __set__
    (instance._meta.object_name, self.related.get_accessor_name()))
ValueError: Cannot assign None: "B.b" does not allow null values.

What’s happening here? Why is A.a acting fine while B.b is unhappy about itself being blank?

---

Edit: I did notice that setting blank=True makes no difference on said behavior, but that still doesn’t explain this issue.

And now this: (?!?!?!)

>>> a = A(a=5)
>>> b = B(b=6)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "django/db/models/base.py", line 357, in __init__
    setattr(self, field.attname, val)
  File "django/db/models/fields/related.py", line 275, in __set__
    self.related.get_accessor_name(), self.related.opts.object_name))
ValueError: Cannot assign "6": "B.b" must be a "B" instance.

Answer 1

OK, I managed to solve this mess.

The problem here is that the model and the field have the same name (case-agnostic). I had this both in my problematic model, as well as the examples here (A.a and B.b).

This is a Django problem in that the error is simply irrelevant to the real issue.

Bottom line – don’t have any field the same name as the model name.