class FavoritePicture(models.Model):
    user = models.ForeignKey(User)
    picture = models.ForeignKey(Pictures)
    created_on = models.DateTimeField(auto_now_add=True)

    class Meta:
        unique_together = ('user', 'picture')

    def __unicode__(self):
    return "%s likes picture %s" % (self.user, self.picture)