title: Django Serializers - Return the sum of values by filter
body: <p>Following this post: <a href="https://stackoverflow.com/questions/53589040/serializer-django-rest-framework-the-serializer-field-might-be-named-incorre/53589433#53589433">Serializer - Django REST Framework - The serializer field might be named incorrectly and not match any attribute or key on the `str` instance</a> </p>
  
  <p>I use this function:</p>
  
  <pre><code>def get_month_hours(self):
      last_year = timezone.now() - datetime.timedelta(days=365)  # for more accurate result, please use python-dateutil or similar tools
      return Timesheet.objects.annotate(total_hour=Sum('working_hour')).filter(owner=self.user, date__gte=last_year).order_by('date')
  </code></pre>
  
  <p>I want to populate the chart who use exactly the same code:</p>
  
  <pre><code>"data": [65, 59, 80, 81, 56, 55, 40],
  </code></pre>
  
  <p>All values corresponding to the total working_hours (my models field) per month.</p>
  
  <p>So, I have 2 problems:
  The first, I receive this data:</p>
  
  <pre><code> {
      "id": 3,
      "title": "Développement intranet",
      "date": "2018-11-19",
      "working_hour": 8.5,
      "week": 47,
      "owner": 1
  },
  {
      "id": 1,
      "title": "Développement intranet",
      "date": "2018-11-26",
      "working_hour": 8.5,
      "week": 48,
      "owner": 1
  },
  {
      "id": 2,
      "title": "Développement intranet",
      "date": "2018-11-27",
      "working_hour": 8.5,
      "week": 48,
      "owner": 1
  },
  {
      "id": 4,
      "title": "dev python",
      "date": "2018-11-28",
      "working_hour": 5.25,
      "week": 48,
      "owner": 1
  },
  {
      "id": 5,
      "title": "Développement intranet",
      "date": "2018-12-03",
      "working_hour": 8.5,
      "week": 49,
      "owner": 1
  }
  </code></pre>
  
  <p>But I want to receive only one value: the sum of working_hour per month.</p>
  
  <p>The second problme is: how to pass this values to my "data" ? I thinked like that:
  "data" : $.ajax({
      url: '/my-json-url/',
      method: 'get',
  }),</p>
  
  <p>right ?</p>
  
  <p>thanks per advance</p>