title: Sum up a field in the queryset in Django
body: <p>models.py</p>
  
  <pre><code>class QaCommission(models.Model):
      user = models.ForeignKey(PlUser, on_delete=models.CASCADE, blank=True, null=True, related_name='user_commission')
      ref = models.ForeignKey(PlUser, on_delete=models.CASCADE, blank=True, null=True, related_name='ref_commission')
      price = models.FloatField(blank=True, null=True)
      pct = models.FloatField(blank=True, null=True)
      commission = models.FloatField(blank=True, null=True)
      status = models.IntegerField(blank=True, null=True, default=0)
  </code></pre>
  
  <p>serializers.py</p>
  
  <pre><code>class QaCommissionSerializer(serializers.ModelSerializer):
      class Meta:
          model = QaCommission
          fields = '__all__'
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>class QaCommissionList(viewsets.ModelViewSet):
      queryset = QaCommission.objects.all()
      serializer_class = QaCommissionSerializer
  </code></pre>
  
  <p>If we are filtering ref=60 in this view, results show like this:</p>
  
  <pre><code>{
  "count": 18,
  "next": "http://127.0.0.1:8008/api/qacommission/?ref=60&amp;page=2",
  "previous": null,
  "results": [
      {
          "id": 1,
          "price": 20.0,
          "pct": 0.1,
          "commission": 2.0,
          "status": 1,
          "user": 7,
          "ref": 60
      },
      {
          "id": 2,
          "price": 10.0,
          "pct": 0.1,
          "commission": 1.0,
          "status": 1,
          "user": 7,
          "ref": 60
      },
      ......
      ......
      ......
      {
          "id": 10,
          "price": 15.0,
          "pct": 0.1,
          "commission": 1.5,
          "status": 1,
          "user": 7,
          "ref": 60
      }
  ]
  }
  </code></pre>
  
  <p>I wanna sum up all the "commission" field in the results and attach the sum to the original queryset (maybe next to "count":18), as shown above, there are 18 commission need to be count.</p>
  
  <p>How could I implement this? Need your help, thanks!</p>