Sanitizing / changing input for serializer fields

[Kevin London]

We're using Django Rest Framework and, in some cases, adjusting the values of the input provided to the serializer based on some other criteria. We're currently doing that in the `validate_field` methods on the serializer and it feels wrong. I'm wondering if there's a better way to approach the problem. For example:

class ExampleSerializer(Serializer):

   name = serializers.CharField()
   is_active = serializers.BooleanField()

   . . .

   def validate_name(self, value):
       name = self.sanitize_input(name)
       if not name:
           raise ValidationError()
       return name

   def validate(self, attrs):
       if attrs['name'] and not attrs['is_active']:
           attrs['is_active'] = True

       return attrs

This is just an example, of course we wouldn't have quite this combination of fields / logic. Is there a better place to do these things like this? Are we approaching it in the wrong way? 

[Chris Foresman]

No, this is pretty much the best way to do it. 

One thing that's not always obvious is that if you're in a view method that uses an existing object, you can access that object via self.instance. So if you're updating an existing instance, you can set values of field in validate_field(self, value) like so:

class ExampleSerializer(Serializer):

   name = serializers.CharField()
   is_active = serializers.BooleanField()

   . . .

   def validate_is_active(self, value):
       if self.instance is not None and self.instance.name:
           return True
       return value

   def validate_name(self, value):

       name = self.sanitize_input(name)
       if not name:
           raise ValidationError()
       return name

   def validate(self, attrs):
       if attrs['name'] and not attrs['is_active']:
           attrs['is_active'] = True

       return attrs