models.py

class Tag(models.Model):
    name = models.CharField(u'Tag', max_length=100)
    post = models.ForeignKey(Post, related_name='tags')
    slug = models.SlugField(u'Slug')

    def get_absolute_url(self):
        return reverse('posts.views.tag', kwargs={'slug': self.slug})

    def __unicode__(self):
        return unicode(self.name)