Django Foreign Key Mismatch

[josephbiko]

Django Foreign key's do not track the models DB when the model is inherited from abstract class.

Steps to reproduce:

create models:"

class A(models.Model):
    field = models.IntegerField()
    
    class Meta:
        abstract = True

class B(A):
    field2 = models.IntegerField()

class C(models.Model):
    fk = models.ForeignKey(B,on_delete=models.CASCADE)

migrate

change the models (by removeing the meta )to:

class A(models.Model):
    field = models.IntegerField()


class B(A):
    field2 = models.IntegerField()

class C(models.Model):
    fk = models.ForeignKey(B,on_delete=models.CASCADE)

run migrations.

Now add an object of class B in the admin.

resulting error:
"foreign key mismatch - "models_c" referencing "models_b""

[Simon Charette]

Hello josephbiko,

I assume you meant that running makemigrations between your two model scenarios doesn't generate the appropriate database alterations?

There's a few bugs (#23521, #25247) related to switching from concrete to abstract inheritance but in the abstract to concrete case there shouldn't be any problem as C.fkshould be automatically repointed to B.a_ptr when A is made concrete.

I'm afraid we'll need a more detailed reproduction test case in order to reproduce your issue. Please include tracebacks and detail every steps your perform to get there.

[Simon Charette]

Upon further inspection I assume you had an existing B entry before attempting to run the migration that adds the B.a_ptr primary key and it failed because no rows were present in the newly added A table?

In this case I think you should be in charge of populating A with a data migration (either using RunPython or RunSQL) instead of expecting Django to do it for you.