text: from django.db import models
  
  
  class R(models.Model):
      is_default = models.BooleanField(default=False)
  
      def __str__(self):
          return "%s" % self.pk
  
  
  def get_default_r():
      return R.objects.get_or_create(is_default=True)[0].pk
  
  
  class S(models.Model):
      r = models.ForeignKey(R, models.CASCADE)
  
  
  class T(models.Model):
      s = models.ForeignKey(S, models.CASCADE)
  
  
  class U(models.Model):
      t = models.ForeignKey(T, models.CASCADE)
  
  
  class RChild(R):
      pass
  
  
  class A(models.Model):
      name = models.CharField(max_length=30)
  
      auto = models.ForeignKey(R, models.CASCADE, related_name="auto_set")
      auto_nullable = models.ForeignKey(R, models.CASCADE, null=True, related_name='auto_nullable_set')
      setvalue = models.ForeignKey(R, models.SET(get_default_r), related_name='setvalue')
      setnull = models.ForeignKey(R, models.SET_NULL, null=True, related_name='setnull_set')
      setdefault = models.ForeignKey(R, models.SET_DEFAULT, default=get_default_r, related_name='setdefault_set')
      setdefault_none = models.ForeignKey(
          R, models.SET_DEFAULT,
          default=None, null=True, related_name='setnull_nullable_set',
      )
      cascade = models.ForeignKey(R, models.CASCADE, related_name='cascade_set')
      cascade_nullable = models.ForeignKey(R, models.CASCADE, null=True, related_name='cascade_nullable_set')
      protect = models.ForeignKey(R, models.PROTECT, null=True)
      donothing = models.ForeignKey(R, models.DO_NOTHING, null=True, related_name='donothing_set')
      child = models.ForeignKey(RChild, models.CASCADE, related_name="child")
      child_setnull = models.ForeignKey(RChild, models.SET_NULL, null=True, related_name="child_setnull")
  
      # A OneToOneField is just a ForeignKey unique=True, so we don't duplicate
      # all the tests; just one smoke test to ensure on_delete works for it as
      # well.
      o2o_setnull = models.ForeignKey(R, models.SET_NULL, null=True, related_name="o2o_nullable_set")
  
  
  def create_a(name):
      a = A(name=name)
      for name in ('auto', 'auto_nullable', 'setvalue', 'setnull', 'setdefault',
                   'setdefault_none', 'cascade', 'cascade_nullable', 'protect',
                   'donothing', 'o2o_setnull'):
          r = R.objects.create()
          setattr(a, name, r)
      a.child = RChild.objects.create()
      a.child_setnull = RChild.objects.create()
      a.save()
      return a
  
  
  class M(models.Model):
      m2m = models.ManyToManyField(R, related_name="m_set")
      m2m_through = models.ManyToManyField(R, through="MR", related_name="m_through_set")
      m2m_through_null = models.ManyToManyField(R, through="MRNull", related_name="m_through_null_set")
  
  
  class MR(models.Model):
      m = models.ForeignKey(M, models.CASCADE)
      r = models.ForeignKey(R, models.CASCADE)
  
  
  class MRNull(models.Model):
      m = models.ForeignKey(M, models.CASCADE)
      r = models.ForeignKey(R, models.SET_NULL, null=True)
  
  
  class Avatar(models.Model):
      desc = models.TextField(null=True)
  
  
  # This model is used to test a duplicate query regression (#25685)
  class AvatarProxy(Avatar):
      class Meta:
          proxy = True
  
  
  class User(models.Model):
      avatar = models.ForeignKey(Avatar, models.CASCADE, null=True)
  
  
  class HiddenUser(models.Model):
      r = models.ForeignKey(R, models.CASCADE, related_name="+")
  
  
  class HiddenUserProfile(models.Model):
      user = models.ForeignKey(HiddenUser, models.CASCADE)
  
  
  class M2MTo(models.Model):
      pass
  
  
  class M2MFrom(models.Model):
      m2m = models.ManyToManyField(M2MTo)
  
  
  class Parent(models.Model):
      pass
  
  
  class Child(Parent):
      pass
  
  
  class Base(models.Model):
      pass
  
  
  class RelToBase(models.Model):
      base = models.ForeignKey(Base, models.DO_NOTHING)
  
  
  class Origin(models.Model):
      pass
  
  
  class Referrer(models.Model):
      origin = models.ForeignKey(Origin, models.CASCADE)
      unique_field = models.IntegerField(unique=True)
      large_field = models.TextField()
  
  
  class SecondReferrer(models.Model):
      referrer = models.ForeignKey(Referrer, models.CASCADE)
      other_referrer = models.ForeignKey(
          Referrer, models.CASCADE, to_field='unique_field', related_name='+'
      )
  