Django. Override save for model

Before saving model I'm re-size a picture. But how can I check if new picture added or just description updated, so I can skip rescaling every time the model is saved?

class Model(model.Model):
    image=models.ImageField(upload_to='folder')
    thumb=models.ImageField(upload_to='folder')
    description=models.CharField()


    def save(self, *args, **kwargs):
        if self.image:
            small=rescale_image(self.image,width=100,height=100)
            self.image_small=SimpleUploadedFile(name,small_pic)
        super(Model, self).save(*args, **kwargs)

I want to rescale only if new image loaded or image updated, but not when description updated.

Solution 1:

Some thoughts:

class Model(model.Model):
    _image=models.ImageField(upload_to='folder')
    thumb=models.ImageField(upload_to='folder')
    description=models.CharField()

    def set_image(self, val):
        self._image = val
        self._image_changed = True

        # Or put whole logic in here
        small = rescale_image(self.image,width=100,height=100)
        self.image_small=SimpleUploadedFile(name,small_pic)

    def get_image(self):
        return self._image

    image = property(get_image, set_image)

    # this is not needed if small_image is created at set_image
    def save(self, *args, **kwargs):
        if getattr(self, '_image_changed', True):
            small=rescale_image(self.image,width=100,height=100)
            self.image_small=SimpleUploadedFile(name,small_pic)
        super(Model, self).save(*args, **kwargs)

Not sure if it would play nice with all pseudo-auto django tools (Example: ModelForm, contrib.admin etc).

Solution 2:

Check the model's pk field. If it is None, then it is a new object.

class Model(model.Model):
    image=models.ImageField(upload_to='folder')
    thumb=models.ImageField(upload_to='folder')
    description=models.CharField()


    def save(self, *args, **kwargs):
        if 'form' in kwargs:
            form=kwargs['form']
        else:
            form=None

        if self.pk is None and form is not None and 'image' in form.changed_data:
            small=rescale_image(self.image,width=100,height=100)
            self.image_small=SimpleUploadedFile(name,small_pic)
        super(Model, self).save(*args, **kwargs)

Edit: I've added a check for 'image' in form.changed_data. This assumes that you're using the admin site to update your images. You'll also have to override the default save_model method as indicated below.

class ModelAdmin(admin.ModelAdmin):
    def save_model(self, request, obj, form, change):
        obj.save(form=form)

Solution 3:

You may supply extra argument for confirming a new image is posted.
Something like:

def save(self, new_image=False, *args, **kwargs):
    if new_image:
        small=rescale_image(self.image,width=100,height=100)
        self.image_small=SimpleUploadedFile(name,small_pic)
    super(Model, self).save(*args, **kwargs)

or pass request variable

def save(self, request=False, *args, **kwargs):
    if request and request.FILES.get('image',False):
        small=rescale_image(self.image,width=100,height=100)
        self.image_small=SimpleUploadedFile(name,small_pic)
    super(Model, self).save(*args, **kwargs)

I think these wont break your save when called simply.

You may put this in your admin.py so that this work with admin site too (for second of above solutions):

class ModelAdmin(admin.ModelAdmin):

    ....
    def save_model(self, request, obj, form, change): 
        instance = form.save(commit=False)
        instance.save(request=request)
        return instance

Solution 4:

What I did to achieve the goal was to make this..

# I added an extra_command argument that defaults to blank
def save(self, extra_command="", *args, **kwargs):

and below the save() method is this..

# override the save method to create an image thumbnail
if self.image and extra_command != "skip creating photo thumbnail":
    # your logic here

so when i edit some fields but not editing the image, I put this..

Model.save("skip creating photo thumbnail")

you can replace the "skip creating photo thumbnail" with "im just editing the description" or a more formal text.

Hope this one helps!

Solution 5:

Query the database for an existing record with the same PK. Compare the file sizes and checksums of the new and existing images to see if they're the same.