lib/django-1.4/tests/modeltests/timezones/models.py

from django.db import models

class Event(models.Model):
    dt = models.DateTimeField()

class MaybeEvent(models.Model):
    dt = models.DateTimeField(blank=True, null=True)

class Session(models.Model):
    name = models.CharField(max_length=20)

class SessionEvent(models.Model):
    dt = models.DateTimeField()
    session = models.ForeignKey(Session, related_name='events')

class Timestamp(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    updated = models.DateTimeField(auto_now=True)