title: changing django model filed data before saving to database
body: <p>i want to update model field data before saving to database.</p>
  <pre><code>class mynumber(TimeStampedModel):
   
   
      text = models.TextField(max_length=10000, null=True, blank=True)
      number = models.DecimalField(default=0, max_digits=9, decimal_places=2)
  </code></pre>
  <pre><code>#serializer file
   
  class mynumberSerializer(serializers.ModelSerializer):
      class Meta:
          model = mynumber
          fields = &quot;__all__&quot;  
  
  </code></pre>
  <pre><code>#view
  class mynumberViewSet(CreateListMixin, mixins.CreateModelMixin, viewsets.GenericViewSet):
   
      serializer_class = mynumberSerializer
      queryset = mynumber.objects.all()
  </code></pre>
  <p>i want to divide number field by 2 before saving to database.
  what should be the best approach?</p>