from django.db import models

class Person(models.Model):

    first_name = models.CharField(max_length=20)

    last_name = models.CharField(max_length=20)

    friends = models.ManyToManyField("self")

# A set of models that use a non-abstract inherited 'through' model.

class ThroughBase(models.Model):

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

    b = models.ForeignKey("B", models.CASCADE)

class Through(ThroughBase):

    extra = models.CharField(max_length=20)

class B(models.Model):

    people = models.ManyToManyField(Person, through=Through)