from django.db import models

class ParentManager(models.Manager):

    def get_by_natural_key(self, parent_data):

        return self.get(parent_data=parent_data)

class Parent(models.Model):

    parent_data = models.CharField(max_length=30, unique=True)

    parent_m2m = models.ManyToManyField("self")

    objects = ParentManager()

    def natural_key(self):

        return (self.parent_data,)

class Child(Parent):

    child_data = models.CharField(max_length=30, unique=True)