Why should I set default value of foreign key, django

Question

For example, we have this code:

class Foo(models.Model):
    a = models.CharField(max_length=10)


class Bar(models.Model):
    b = models.CharField(max_length=10)
    a = models.ForeignKey(Foo)

And when I run makemigrations, i see that I need set default of a(Bar) Can anyone explain me, why should I do it?

Thanks!

Answer 1

It seems that you already have some data in your database (or you have made a migration before adding the ForeignKey field). In order for the migration to work you will need to "tell" Django what data should be in the a column of the Bar table for rows that already exist.

Either delete your whole database/earlier migrations, or provide a default value of your choice:

a = models.ForeignKey(Foo, default=None) # or any other default value