Set model field default to value from foreign key instance

Question

Working on a Django app with two models (A and B), B has a field link which is a foreign key relationship to A:

# models.py
class A(models.Model):
    name = models.CharField(max_length=100)
    description = models.CharField(max_length=15)
    my_bool = models.BooleanField(default=True)

class B(models.Model):
    link = models.ForeignKey(A)
    b_bool = models.BooleanField(default=link.my_bool) # Error!

I would like for the b_bool field to have the linked my_bool value as a default if no B.b_bool is provided via graphene mutation.

Currently, using link.my_bool as a default raises the following error when making migrations:

AttributeError: 'ForeignKey' object has no attribute 'my_bool'

Answer 1

I don't think it will work like that. Instead, try overriding save() method:

class B(models.Model):
    link = models.ForeignKey(A)
    b_bool = models.BooleanField(default=False)

   def save(self, *args, **kwargs):
     if not self.b_bool:
         self.b_bool = self.link.my_bool
     super(B, self).save(*args, **kwargs)