class Article(models.Model):
    title = models.CharField(max_length=100)
body = models.TextField()


class Cast(models.Model):
    title = models.CharField(max_length=100)
body = models.TextField()
FAVE_CHOICES = ('A', 'Article'), ('C', 'Cast')


class Fave(models.Model):
    type_of_fave = models.CharField(max_length=1, choices=FAVE_CHOICES)
cast = models.ForeignKey(Casts, null=True)
article = models.ForeigKey(Articles, null=True)
user = models.ForeignKey(User, unique=True)
Как использовать динамический внешний ключ в Django?