title: Django - update another model on save in admin
body: <p>I am using django to create a video library website.</p>
  
  <p>I have 3 models for the library: Video, VideoTopic, and Course.</p>
  
  <p>I have a TimedeltaField for the duration of each Video.  Upon saving each video in admin, I want to get the sum of each TimedeltaField for all Videos within a Course, then save that sum to a TimedeltaField in my Course model.</p>
  
  <p>I'm using <a href="https://pypi.python.org/pypi/django-timedeltafield" rel="nofollow">django-timedeltafield</a>, which will handle the summarization very well.  What I have yet to find a solution for is how to update the Course model after I save each Video in admin.  Any advice?</p>
  
  <p>My models are as follows:</p>
  
  <pre><code>class Video(models.Model):
      title = models.CharField(max_length=200)
      topic = models.ForeignKey('VideoTopic')
      duration = timedelta.fields.TimedeltaField(default="3 minutes, 30 seconds", help_text="(x minutes, x seconds)")
  
  class VideoTopic(models.Model):
      name = models.CharField(max_length=200, blank=True)
      course = models.ForeignKey('course.Course')
  
  class Course(models.Model):
      name = models.CharField(max_length=200)
      duration = timedelta.fields.TimedeltaField()
  </code></pre>