How do I prevent Django from saving an Image each time a user submits a custom UserChangeForm?

Question

So, I'm a newb in Django and I've defined a custom user, users are allowed to update their profiles using a ModelForm however each time I submit the form it uploads and proceses the thumbnail again, how can I check if the upload field has changed and exclude it from the submission if it has not?

here is my code:

views.py

@login_required
def account_edit(request):
    u = AccountUser.objects.get(pk=request.user.id)
    if request.method == 'POST':
        form = UserChangeForm(request.POST, request.FILES, instance=request.user)
        if form.is_valid():
            form.save()
    else:
        form = UserChangeForm(instance=request.user)
    return render_to_response('accounts/account.html', locals(), RequestContext(request))

models.py

class AccountUser(AbstractUser):
    about = models.TextField(blank=True, null=True)
    occupation = models.CharField(max_length=255, blank=True, null=True)
    avatar = models.ImageField(upload_to="accounts/avatars", blank=True, null=True)
    slug = models.SlugField(default='', blank=True)
    following = models.ManyToManyField('AccountUser', related_name="followed_by", blank=True, null=True)
    thumbnail = models.ImageField(upload_to="accounts/avatars/thumbnails", blank=True, null=True)

    def save(self, *args, **kwargs):
        if self.avatar is not None:
            img = Image.open(self.avatar.file)
            thumbnail = ImageOps.fit(img, (220, 220,), method=Image.ANTIALIAS)


            temp_handle_img = StringIO()
            img.save(temp_handle_img, 'png')
            temp_handle_img.seek(0)

            temp_handle_thumbnail = StringIO()
            thumbnail.save(temp_handle_thumbnail, 'png')
            temp_handle_thumbnail.seek(0)

            suf_thumbnail = SimpleUploadedFile(os.path.split(self.avatar.name[-1],temp_handle_thumbnail.read(), content_type='image/png')
            fname_thumbnail = "%s.png" % os.path.splitext(self.avatar.name)[0]

            sufImg = SimpleUploadedFile(os.path.split(self.avatar.name)[-1], temp_handle_img.read(), content_type='image/png')
            fnameImg = "%s.png" % os.path.splitext(self.avatar.name)[0]

            self.thumbnail.save(fname_thumbnail, suf_thumbnail, save=False)
            self.avatar.save(fnameImg, sufImg, save=False)
        if not self.slug:
            self.slug = slugify(self.username)
        super(AccountUser, self).save(*args, **kwargs)

Answer 1

You should remove 'thumbnail' from your model fields and set it as a def . thumbnail doesn't seem to need to be in db.

Edit:

I think, you should hash your picture for see if changed. Use hashlib.md5 for example.

def save(self, *args, **kwargs):
    if self.avatar is not None:
        # Do your stuff
        cur_file = AccountUser.objects.get(pk=self.pk).avatar.file
        cur_file_hash = md5(cure_file.read()).hexdigest()
        new_file_hash = md5(temp_handle_img.read()).hexdigest()
        if cur_file_hash != new_file_hash:
            # Do some stuff

I don't think I exactly resolve it, but it should give you a way...