When to Override Django Model Saving for Image Resizing

Overriding Django Model Saving for Specific Cases

In Django, the save() method in models is responsible for persisting data to the database. However, there are scenarios where you may want to modify this behavior based on specific criteria. One such case is rescaling an image only when it is newly added or updated.

Problem:

In a model with fields for image, thumb, and description, you need to resize and store a thumbnail (thumb) only if the main image (image) is created or modified. Updating the description field should not trigger image resizing.

Solution:

To achieve this, you can override the default save() method by using a custom property that checks for changes in the image field:

<code class="python">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

    def get_image(self):
        return self._image

    image = property(get_image, set_image)

    def save(self, *args, **kwargs):
        if getattr(self, '_image_changed', True):
            small = rescale_image(self.image, width=100, height=100)
            self.thumb = SimpleUploadedFile(name, small_pic)
        super(Model, self).save(*args, **kwargs)</code>

How it Works:

• The set_image() method updates the _image field and sets a flag _image_changed to True.
• The get_image() method returns the current value of the _image field.
• The image property wraps both the set_image() and get_image() methods to provide a consistent interface.
• In the overridden save() method, the _image_changed flag is checked. If it's True, it means the image has been changed or added.
• In that case, the thumbnail is rescaled.
• The super() method is called to save the model to the database with the rescaled thumbnail (if necessary) and updated description.

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