title: Sum of Duration Fields in Django
body: <p>I am having a small issue in summing up two simple duration fields.
  I have two variables which contains two different duration fields. Basically, I just need to sum them in order to get the total time. The problem is that, the way the database is built, I can't use Sum or F or ExpressionWrapper because I have to accept also None values and, the calculation gives me a None value in return.
  I post some code:</p>
  <p>views. py</p>
  <pre><code>duration_dual = Mission.objects.filter(
      training_course_id=1, solo_flight=False)
  total_dual_duration = duration_dual.aggregate(eet=Sum(ExpressionWrapper(
      F('duration_dual'), output_field=IntegerField()), output_field=DurationField()))['eet']
  if total_dual_duration != None:
      total_dual_duration = duration(total_dual_duration)
  
  duration_solo = Mission.objects.filter(
          training_course_id=1, solo_flight=True)
      total_solo_duration = duration_solo.aggregate(eet=Sum(ExpressionWrapper(
          F('duration_solo'), output_field=IntegerField()), output_field=DurationField()))['eet']
      if total_solo_duration != None:
          total_solo_duration = duration(total_solo_duration)
  </code></pre>
  <p>models.py</p>
  <pre><code>class Mission(models.Model):
  name = models.CharField(max_length=200)
  duration_dual = models.DurationField(blank=True, null=True)
  duration_solo = models.DurationField(blank=True, null=True)
  training_course = models.ForeignKey(
      TrainingCourse, on_delete=models.CASCADE)
  note = models.TextField(null=True, blank=True)
  solo_flight = models.BooleanField(default=False)
  created_at = models.DateTimeField(auto_now_add=True)
  updated_at = models.DateTimeField(auto_now=True)
  </code></pre>
  <p>In fact, what I need to do is just adding the total_solo_duration and total_dual_duration variables. I tried with datetime or timedelta but I can't figure out the proper way to do so.
  Thank you very much in advance</p>