title: Django schedule a "one time" task based on datetime model attribute
body: <p>What the best solution in Django to solve this kind of problem:</p>
  
  <p>I need to set a schedule time, based on an object attribute value, to run a "one time" task, when schedule time is reached.</p>
  
  <p>For each attribute updates, the schedule time has to be also updated.</p>
  
  <p><strong>Example (pseudo code)</strong></p>
  
  <pre><code>class Runner(models.Model):
      execute_time = models.DateTimeField()
  
  post_save( update_scheduler, sender=Runner)
  
  
  def update_scheduler(sender, instance, created, **kwargs):
      if created:
          # set schedule time = instance.execute_time
          create_or_update_schedule(instance.datetime)
  </code></pre>
  
  <p>Is it possibile to do something like this using Celery? update schedule time on object update?</p>