title: django dynamic FilePathField
body: <pre><code>class Project(models.Model):
      slug = models.SlugField(max_length=100)
      main_file = models.FilePathField(path="/home/mn/myfiles/%s" % slug)
  </code></pre>
  
  <p>This code doesn't work, just illustrating what I'd like to do. 
  I want to fill in <code>FilePathField</code> when object is accessed (ie. when it is updated in django admin, not when it's created).</p>
  
  <p>Is there a hook where you can set fields just before they are filled? Something like <code>object.on_get(instance):</code> ... perhaps?</p>