Regarding Base and Child Models: Django - django

Question

I have a scenario in which I want the application to fail at the compile/build time if the child model does not have sub_field_a.
Only models which have sub_field_a should be able to inherit from Base model otherwise the application should fail at compile/build time.
class Base(models.Model):
class Meta:
abstract = True
class SubA(Base):
sub_field_a = models.CharField(max_length=255)
sub_field_c = models.CharField(max_length=255)
class SubB(Base):
sub_field_b = models.CharField(max_length=255)
sub_field_c = models.CharField(max_length=255)
Here, I should get an error because SubB does not have sub_field_a
How can I achieve this? TIA

Answer 1

Assuming you are using Django 1.7 or above, you should be able to use the system check framework; specifically, the field, model and manager checks.
I believe it would look something like this:
from django.core import checks
from django.db import models
class Base(models.Model):
class Meta:
abstract = True
#classmethod
def check(self, **kwargs):
errors = super(Base, self).check(**kwargs)
if not self._meta.abstract and not hasattr(self, 'sub_field_a'):
errors.extend([
checks.Error(
'missing sub_field_a',
hint='Add a field named sub_field_a',
obj=self,
id='myapp.E001',
)
])
return errors

Answer 2

Isn't the whole point of inheritance is to reduce and reuse common code. In this case we know sub_field_a always need to be defined. In that case why not add it to the base model like this:
class Base(models.Model):
sub_field_a = models.CharField(max_length=255)
class Meta:
abstract = True
class SubA(Base):
sub_field_c = models.CharField(max_length=255)
class SubB(Base):
sub_field_b = models.CharField(max_length=255)
sub_field_c = models.CharField(max_length=255)
This totally invalidates the use case that is required here. Hope this helps :)