class UploadedImage(models.Model):
    image = models.ImageField(unique=True)
    input_url = models.URLField(blank=True)
    image_hash = models.CharField(unique=True, max_length=32)
    created_time = models.DateTimeField(auto_now_add=True)

    def save(self, *args, **kwargs):
        hash_sha1 = hashlib.sha1()
        for chunk in self.image.chunks():
            hash_sha1.update(chunk)
        self.image_hash = hash_sha1.hexdigest()
        super(UploadedImage, self).save(*args, **kwargs)