from django.db import models

class Person(models.Model):

    name = models.CharField(max_length=200)

class Movie(models.Model):

    title = models.CharField(max_length=200)

    director = models.ForeignKey(Person, models.CASCADE)

class Event(models.Model):

    pass

class Screening(Event):

    movie = models.ForeignKey(Movie, models.CASCADE)

class ScreeningNullFK(Event):

    movie = models.ForeignKey(Movie, models.SET_NULL, null=True)

class Package(models.Model):

    screening = models.ForeignKey(Screening, models.SET_NULL, null=True)

class PackageNullFK(models.Model):

    screening = models.ForeignKey(ScreeningNullFK, models.SET_NULL, null=True)