ETag "ready" Django model using a version number

example_model.py

class Song(VersionedModel):
    """
    >>> song = Song(title='Here Come the Sun')
    >>> song.save()
    >>> cache.get('music.song-1,1)
    1
    >>> song.title = 'Here Comes the Sun'
    >>> song.save()
    >>> cache.get('music.song-1,1)
    >>> cache.get('music.song-1,2)
    1
    """
    title = models.CharField(max_length=100)

versioned_model.py

from django.db import models
from django.core.cache import cache

class VersionedModel(models.Model):
    version = models.IntegerField(default=0)

    class Meta(object):
        abstract = True

    def save(self, *args, **kwargs):
        etag = self.invalidate_cache()
        self.version += 1
        try:
            super(VersionedModel, self).save(*args, **kwargs)
        except Exception, e:
            # Revert if the save failed
            self.version -= 1
            cache.set(etag, 1)
            raise e

    def invalidate_cache(self):
        etag = self.generate_etag()
        cache.delete(etag)
        return etag

    def generate_etag(self):
        kwargs = {
            'app': self._meta.app_label,
            'model': self._meta.module_name,
            'id': self.id,
            'version': self.version,
        }
        return '{app}.{model}-{id},{version}'.format(**kwargs)