title: Django ImageField passing a callable to upload_to
body: <p>I'm trying to pass a custom upload_to function to my models imageField but I'd like to define the function as a model function....is that possible?</p>
  
  <pre><code>class MyModel(models.Model):
      ...
      image = models.ImageField(upload_to=self.get_image_path)
      ...
  
      def get_image_path(self, filename):
          ...
          return image_path
  </code></pre>
  
  <p>Now i know i can't reference it by 'self' since self doesn't exist at that point...is there a way to do this? If not - where is the best place to define that function?</p>