Replacing CharField with a ForeignKey based on the CharField - django

Question

While refactoring an app, I've started removing a config file and replacing it with a new model, Foo in this example. Because of this, another model, Bar in this example, needs to change from having a CharField that was for the config file, to a ForeignKey that is for the new model.
Say I have
class Foo(models.Model):
name = models.CharField(primary_key=True, max_length=100)
class Bar(models.Model):
name_of_foo = models.CharField(max_length=100)
and I want to change name_of_foo to instead be a ForeignKey...
class Bar(models.Model):
foo = models.ForeignKey(Foo, on_delete=models.CASCADE, default=???)
I would like the default to be based on what name_of_foo was. If name_of_foo was "abc", I would like it to do something akin to default=Foo.objects.get("abc").
Is there a way to fill in the ??? such that this works nicely? If not, what steps can I take to arrive here, so that the existing data is converted?

Answer 1

I'm not sure if it is possible to achieve what you want using the default field but I'll leave it to someone else with more knowledge than me to answer that.
Assuming that a Foo object exists for each Bar object where foo.name = bar.name_of_foo you should be able to add a foreign key to Bar that links to the Foo object with the following steps.
Add foreign key field with null=True
class Bar(models.Model):
name_of_foo = models.CharField(max_length=100)
foo = models.ForeignKey(Foo, on_delete=models.CASCADE, null=True)
run makemigrations and migrate commands
Open the django shell
python manage.py shell
run the following code to update the foreign key of each Bar object (making changes where necessary)
from <app_name>.models import Foo, Bar
for foo in Foo.objects.all():
bar = Bar.objects.get(name_of_foo=foo.name)
bar.foo = foo
bar.save()
remove name_of_foo field
class Bar(models.Model):
foo = models.ForeignKey(Foo, on_delete=models.CASCADE, null=True)
run makemigrations and migrate commands