Django: Model Inheritance: FK & M2M

Question

I dam trying to do this: http://docs.djangoproject.com/en/dev/topics/db/models/#be-careful-with-related-name

Using this style

This is saved as common/abstract.py

class OtherModel(models.Model):

   something = Charfield(max_length=100)

   class Meta:
            abstract = True

class Base(models.Model):
        fk_model = models.ForeignKey(OtherModel, related_name="%(app_label)s_%(class)s_related")

        class Meta:
            abstract = True

Then I import it into another file. Let's call this app/models.py

from common.abstract import Base

class ChildB(Base):
    pass

I have installed 'app' but not the 'common'. It will import nicely without the FK or M2M relationship, but when I try to add it, I get this error:

/lib/python2.6/site-packages/django/db/models/fields/related.py", line 808, in init assert not to.meta.abstract, "%s cannot define a relation with abstract class %s" % (self.class._name__, to._meta.object_name) AssertionError: ForeignKey cannot define a relation with abstract class OtherModel

Please advise.... also let me know if you have any questions or don't understand something that I am explaining. The file that I working with is really complex, so I didn't want to post the whole thing since I knew that it is breaking on this one relationship.

Answer 1

You cannot have a relation with an abstract model, but this is what you are trying to do (ManyToManyField to OtherModel). If you want to make this work you need to remove abstract = True from OtherModel and add common to your INSTALLED_APPS!

If you want to relate to different sublasses of OtherModel from the subclasses of Base you will need to define the relation on the subclass, not on the abstract model!

Answer 2

I figured out the problem.....

This will work:

from django.db import models

class CommonInfo(models.Model):

    name = models.CharField(max_length=100)

class Base(models.Model):
    m2m = models.ForeignKey(CommonInfo, related_name="%(app_label)s_%(class)s_related")

    class Meta:
        abstract = True

class ChildA(Base):
    pass

This will give you the error: AssertionError: ForeignKey cannot define a relation with abstract class OtherModel.This is because you can't have a FK relationship or M2M relationship on an abstract pointing to ANOTHER abstract class.

from django.db import models

class CommonInfo(models.Model):
    class Meta:
        abstract = True

    name = models.CharField(max_length=100)

class Base(models.Model):
    m2m = models.ForeignKey(CommonInfo, related_name="%(app_label)s_%(class)s_related")

    class Meta:
        abstract = True

class ChildA(Base):
    pass

This is sad news for me. Sad news indeed. I hope my sad news benefits someone else.