title: Django. Override save for model
body: <p>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?</p>
  
  <pre><code>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)
  </code></pre>
  
  <p>I want to rescale only if new image loaded or image updated, but not when description updated.</p>