django-model-utils/model_utils/models.py

from django.db import models
from django.contrib.contenttypes.models import ContentType

from model_utils.fields import AutoCreatedField, AutoLastModifiedField

class InheritanceCastModel(models.Model):
    """
    An abstract base class that provides a ``real_type`` FK to ContentType.

    For use in trees of inherited models, to be able to downcast
    parent instances to their child types.

    """
    real_type = models.ForeignKey(ContentType, editable=False, null=True)

    def save(self, *args, **kwargs):
        if not self.id:
            self.real_type = self._get_real_type()
        super(InheritanceCastModel, self).save(*args, **kwargs)

    def _get_real_type(self):
        return ContentType.objects.get_for_model(type(self))

    def cast(self):
        return self.real_type.get_object_for_this_type(pk=self.pk)

    class Meta:
        abstract = True


class TimeStampedModel(models.Model):
    """
    An abstract base class model that provides self-updating
    ``created`` and ``modified`` fields.

    """
    created = AutoCreatedField()
    modified = AutoLastModifiedField()

    class Meta:
        abstract = True