class ModelA(models.Model):
    field1 = models.ForeignKey('ModelB')
    field2 = models.CharField(max_length=400)

class ModelB(models.Model):
    field1 = models.ForeignKey('ModelA')
    field2 = models.CharField(max_length=400)

class ModelC(models.Model):
    field1 = models.ForeignKey('ModelA')
    field2 = models.IntegerField()

class ModelD(models.Model):
    field1 = models.CharField(max_length=400)