Handling Two Tables with Circular Foreign Keys

I encountered this question in an interview:

How should you handle two tables whose primary keys are foreign keys to each other?

Option 1

This approach is suitable when working directly in the database:

set @@foreign_key_checks=OFF

Option 2

A foreign key may be null. When using an ORM, allow the relevant foreign-key fields to be null. Save one model first with a null foreign key, save the other model, then update the first model’s foreign key.

For example, in Django:

from django.db import models

class Action(models.Model):
    step = models.ForeignKey('Step', related_name='step', null=True, blank=True)

class Step(models.Model):
    action = models.ForeignKey('Action', related_name='action', null=True, blank=True)

Save the objects in this order:

a = Action(step_id=None)
a.save()

s = Step(action_id=a.id)
s.save()

a.step_id = s.id
a.save()

Originally published on SegmentFault.