'Ensure that register delete rule adds a delete rule to the document
meta.'

def test_register_delete_rule(self):

    class Job(Document, ):
        employee = ReferenceField(self.Person)
    self.assertEqual(self.Person._meta.get('delete_rules'), None)
    self.Person.register_delete_rule(Job, 'employee', NULLIFY)
    self.assertEqual(self.Person._meta['delete_rules'], {(Job, 'employee'): NULLIFY})
