how to use django Serializer to update an instance

Question

in Django PUT method, I want to update an instance:

sv= SV.objects.get(pk=pk)
serializer = SVSerializer(sv, data=request.data)
if serializer.is_valid():

Here, in request.data, I just want to pass some of the variable of SV. But as some fields missing, the is_vaild will be false. What I want is, just update the fields in request.data, for the other ones, keep the value in sv. How could I do that?

Answer 1

Perform a partial update by setting partial=True:

sv= SV.objects.get(pk=pk)
serializer = SVSerializer(sv, data=request.data, partial=True)
if serializer.is_valid():
    serializer.save()
else:
    # Do something else

This allows a PATCH request.

Edit

If you want a default field during partial update (as requested in a comment) override the update method:

SVSerializer(serializers.ModelSerializer): # Instead of exposing the 
    state_flag = serializers.SlugRelatedField(source='sv_state', queryset=SVState.objects.all(), slug_field='flag')
    
    def update(self, instance, validated_data):
        if self.partial and validated_data.get('state_flag') == None:
            validated_data['state_flag'] = 0
        super().update(instance=instance, validated_data=validated_data)