/tests/modeltests/mutually_referential/tests.py

from django.test import TestCase
from models import Parent, Child

class MutuallyReferentialTests(TestCase):

    def test_mutually_referential(self):
        # Create a Parent
        q = Parent(name='Elizabeth')
        q.save()

        # Create some children
        c = q.child_set.create(name='Charles')
        e = q.child_set.create(name='Edward')

        # Set the best child
        # No assertion require here; if basic assignment and
        # deletion works, the test passes.
        q.bestchild = c
        q.save()
        q.delete()